♦ LIBRARY OF CONGRESS.? 



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! UNITED STATES OP AMERICA, i 





ectruiD showing eke absorptive power of Sodium vapour Pig.6 




THE 



PHENOMENA AND LAWS 



OF 



HEAT. 



/ 



By ACHILLE CAZIN, 

PROFESSOR OF PHYSICS IN THE LYCEUM OF VERSAILLES. 



% xnvalnttb mtir (Biriteb bg 
ELIHU RICH, 

EDITOR OF GRIFFIN'S "CYCLOPAEDIA OF BIOGRAPHY," AND " OCCULT SCIENCES," 
LATE EDITOR OF " THE PEOPLE'S MAGAZINE," ETC. ETC- 



•i 




NEW YORK: 
SCRIBNER, ARMSTRONG & CO., 

SUCCESSORS TO 

CHARLES SCEIBNER & CO., 

654 BROADWAY. 

1872. 



Itenogf*^ 



Q.eO-55 



TRANSLATOR'S PREFACE 



The aim of this little book is to present in a summary 
the principal phenomena of heat, as viewed from the 
stand-point afforded by recent discoveries in Physics. 
Professor Tyndall has already published a work of 
authority on the subject, under the title of Heat considered 
as a Mode of a Motion, the translation of which, into French, 
by M. the Abbe Moigno, has very much contributed to 
the diffusion of the new ideas on the Continent. To this 
important work, M. Cazin acknowledges he is indebted 
for much valuable assistance in the composition of the 
following popular sketch. He has thought it necessary, 
however, to caution his readers against what he calls a 
" difficulty," in the following words : — 

" The kind of reasoning adopted by many authors who 
have written on the mechanical theory of Heat, might 
lead one to suppose they were the disciples of some 
peculiar school of Philosophy > and that they had bor- 
rowed from certain metaphysical doctrines the principles 
they advance. Nothing could be further from the truth 
than such an opinion. When physicists affirm that the 
phenomena of heat are due to certain movements of the 
particles of matter, they simply express a fact experi- 
mentally known to them, and concerning which there can 



IV PREFACE. 

be no manner of doubt. It is no part of their business, 
and they do not pretend to deduce from the correlation 
they observe between heat and the sensible or atomic 
movements of bodies, any conclusion relative to the 
constitution of the universe. Nothing could be more 
unfair than to allege that their opinions lead to the 
negation of a primal force, and thus to materialism. 
A distinguished writer on Physics, M. Hirn, has even 
demonstrated, in his Exposition de la Theorie mecanique 
de la Chaleur, that the experimental principles upon which 
they depend result logically, neither in materialism nor 
pantheism, but in the purest spiritualism. The essential 
character of the new theory of heat is to show the con- 
nexion of phenomena, in a continued chain, viewed 
independently of their causes; that is to say, without 
taking into consideration the nature of the forces which 
produce them." 

The reader of this little Treatise, therefore, will not 
expect to find anything in its pages but a simple account 
of phenomena ; and he is requested to bear in mind that 
the first cause of heat is never brought into question. 
With regard to the translation, it is only fair to observe 
that it is never easy to render a technical work from one 
language into another. To secure accuracy, the writer 
has availed himself of the professional knowledge of his 
son, whose scientific aptitude encourages the hope that 
he may hereafter take the responsibility of more important 
work. 

Muswell Hill, 

ioM August 1868. 



CONTENTS. 



CHAPTER I. 



GENERAL PHENOMENA OF HEAT. 



PAGE 



1. Our ' ordinary perceptions of Heat distinguished from its 

proper phenomena . . • . . . I 

2. Heat produced by the atomic movement which produces 

chemical action * 2 

3. Transference of Heat by radiation ..... 4 

4. Transference of Heat by conduction . . . . . 5 

5. The combustion of bodies determined by Heat ... 5 

6. Change of bulk effected by Heat 7 

7. Fusion and solidification 8 

8. Evaporation, ebullition, and condensation of vapours . 9 

9. Mechanical effects of Heat . . . . . . . 1 1 

10. A fire-machine spends Heat in effecting work . . . 15 

11. Relation between animal Heat and mechanical work . 19 

12. Electricity produced by Heat 22 

13. Fire-worshippers 24 



CHAPTER II. 



ON THE EXPERIMENTAL METHOD AND THE THERMOMETER. 



1. Aim of physics 

2. Hypothesis as to the nature of Heat 

3. Invention of the thermometer 

4. Graduation and use of the thermometer 

5. The measurement of Heat . 



26 
28 
32 
36 
39 



VI CONTENTS. 

CHAPTER III. 

SOURCES OF HEAT. 

PAGE 

1. Solar Heat. — Terrestrial Heat 42 

2. Heat produced by chemical action. — Combustion . . 44 

3. Heat produced by mechanical motion 54 

4. Heat produced by mechanical work is equivalent to the work 

done . 56 

5. Mechanical equivalent of Heat 60 

6. On some grand natural phenomena in which mechanical force 

is converted into Heat . . 63 

7. Heat developed by electricity 69 

8. Heat developed by animal and vegetable life . 70 

• 

CHAPTER IV. 

' THE RADIATION OF HEAT. 

i. Reflection of Heat. — Burning-mirrors . c • 

2. Refraction of Heat. — Burning-glasses .... 

3. Physical identity of a ray of light with a ray of Heat- 

Luminous and calorific spectra .... 

4. Sifting of the rays 

5. Influence of temperature on the emission of Heat 

6. Influence of the nature of the source on emission. — Corre 

lation between emission and absorption 

7. Influence of distance on radiant Heat 

8. Various applications of the preceding principles. — Dew. — 

Atmospheric water vapour 100 



85 
92 

95 

97 
99 



CHAPTER V. 

CONDUCTION OF HEAT. 

1. Bodies which are good conductors . 107 

2. Substances which are bad conductors . . , .114 

3. - Convection of Heat in liquids and gases . . , .117 

4. Effect* of convection in the ocean. — Marine currents , , 122 

5. Effects of convection in the atmosphere. — Winds . . .129 



CONTENTS. Vli 



CHAPTER VI. 



CHANGE OF THE VOLUME OF BODIES. 

PAGE 

1. Action of Heat on gases. — Sensible Heat. — Exterior work . 133 

2. Action of Heat on solids and liquids. — Grandeur of mole- 

cular forces. — Interior work . . . . . .141 

3. How the expansion of bodies is measured. — Maximum density 

of water . 146 

4. Explanation of various phenomena . . . . .152 

5. On specific Heat 155 



CHAPTER VII/ 

ON FUSION AND SOLIDIFICATION. 

1. Law concerning the temperature at which substances fuse. — 

Heat consumed during fusion and produced by solidifica- 
tion 158 

2. Interior work. — Crystals. — Ice-flowers . . . ' . 163 

3. Change in volume and exterior resistance. — Expansive force 

oMce 170 

4. Regelation. — Glaciers 175 



CHAPTER VIII. 



CONCERNING EVAPORATION AND EBULLITION. 

1. Superficial evaporation of solids and liquids . . . 1 81 

2. Elastic force of vapours. — Papin's Digester . . . 184 

3. Evaporation is accompanied bv a disappearance of sensible 

Heat x . . . . 192 

4. Ebullition under a constant pressure. — Law of temperature . 194 

5. Interior and exterior work. — Heat of evaporation . . 198 

6. The utilization 01 Heat in the steam-engine and hot-air engine 202 

7. How a liquid may be made to boil by cold .... 204 

8. The Geysers 208 

9. The spheroidal state of liquids. — How the human body may 

be incombustible 212 



Vffi CONTENTS. 



CHAPTER IX. 

ON THE THREE STATES OF MATTER, AND ON THE ARTIFICIAL 
METHODS OF PRODUCING COLD. 

PAGE 

1. Liquefaction of gases, and solidification of liquids . . 220 

2. Artificial production of cold. — Manufacture of ice. — Solid 

carbonic acid . ........ 225 

3. Solution. — Crystallisation 239 



CHAPTER X. 

HEAT UPON THE TERRESTRIAL GLOBE. 

1. Equilibrium of Heat on the surface of the globe. — Law of the 

conservation of energy . . . . . . 243 

2. Distribution of temperature on the earth's surface. — Climates, 

— Effects of atmospheric moisture . . . . 249 

3. The changes which have occurred in the distribution of Heat 

previous to our epoch. — Geological revolutions . . 257 

4. Future of the terrestrial globe •••••• 264 



LIST OF ILLUSTRATIONS 



KG. PAGff 

1. Recoiling ^Eolypile II 

2. Laubereau's Hot-air Engine . . . • . . 13 
2<z.Section of the same Engine ..••.. 15 

3. Haiiy's Tourmaline Apparatus ...... 22 

4. Thermo-electric Pile 23 

5. Pile and Galvanometer 23 

6. Air Thermometers 33 

7. Alcohol Thermometer ....... 34 

8. Apparatus for graduating the Thermometer . • . 37 

9. Ordinary Thermometers . . . . . . . 37 

10. Brongniart's Pyrometer 38 

11. Pyrheliometer . . . 43 

12 Combustion of Phosphorus in Chlorine .... 46 

13. Gas Flame 48 

14. Bunsen's Gas Burner ........ 5° 

15. Candle Flame 50 

16. Oxy-hydiogen Blowpipe . .' 53 

17. Compressibility of liquids • 55 

18. Compression Syringe . ...... 56 

19. Joule's Apparatus for producing Heat by the friction of 

liquids .... ..... 62 

20. Theory of Tides 66 

21. Voltaic Pile and Arc . . . . . . . 71 

22. Spathe of the Arum ...... • • 73 

23. Burning-mirror . .. . . . . . . . 76 

24. Two opposed Mirrors . ....... 78 

25. Converging Lens 81 

25<?.Bernieres' Immense Burning-glass 83 

26. Lens 85 

27. Savart's Wheel . . . . . . . . 87 

28. Savart's Wheels 89 



LIST OF ILLUSTRATIONS. 



FIG. PAGE 

29. Solar Spectrum {Frontispiece.) 

30. Successive Prisms showing the action of the Prism on simple 

rays {Frontispiece.) 

31. Inverted Prisms ......... 95 

32. Leslie's Cube ....,..*. 97 

33. Voltaic Arc . . 98 

34. Spectrum of Sodium {Frontispiece.) 

35. Conductibility of solid bodies 10S 

36. Ingenhous's Apparatus 109 

37. Conductibility of Iron and Bismuth . . ...... 1 10 

38. Double Cylinder of copper and wood . , . . .Ill 

39. Properties of Metallic Gauze . . . . .112 

40. Sir Humphrey Davy's Safety Lamp 113 

41. Ice-house . . . . . .. . . .115 

42. Convection of water 117 

43. Apparatus to demonstrate the conductibility of liquids . 119 

44. Water boiling over ice . ,. . . . . .. 120 

45. Apparatus for showing the cooling powers of gases . . 121 

46. Theory of Marine Currents . . . . 123 

47. Map of Marine Currents 125 

48. Theory of Polar Currents .129 

49. Expansion of air under constant pressure . . . 134 

50. Montgolfier's Balloon . . . . 135 

51. Heating of a gas, its volume remaining the same . . 138 

52. Straightening of the walls at the Conservatoire des arts et 

Metiers 144 

53. Apparent Expansion of Liquids 147 

54. Apparatus for measuring the Expansion of Solids . . 1 50 

55. Double bar (iron and copper) . . . . . .152 

56. Trevelyan's Experiment . . . . . . 153 

57. Gore's Experiment 154 

58. Fusion of Sulphur 159 

59. Crystallization of Sulphur 164 

60. Various forms of Snow . . . . . .166 

61. Dissection of Ice by the Solar Rays . . . . . 166 

62. Ice Flowers . ....... 167 

63. Exhibition of Ice Flowers by projection . . . .170 

64. Expansive force of Ice 173 

65. Freezing of Water in an open vessel 174 

66. Freezing of Water above an uneven surface . . .175 

67. Moulding Ice . .176 

68. Air-cells in Ice 178 

69. Apparatus for the vaporisation of liquids . . . .185 

70. Papin's Digester . . . ."" 188 

71. Ordinary Ebullition 194 

72. Donny's experiment in Ebullition . . . . .196 

73. Still 201 



LIST OF ILLUSTRATIONS, XI 



FIG. 

74. Ebullition in vacuo 

75. Ebullition of water by cold 

76. Theory of Geysers ....... 

77. Spheroidal State of Water ...... 

78. Flame seen between the hot surface and the globule , 

79. Explosion produced by the Cooling of Water in the sphe 

roidal state .... .... 

80. Liquefaction of Sulphurous Acid by Cold . 

81. Liquefaction of Sulphurous Acid by pressure . . 

82. Liquefaction of Gases by the process of Davy and Faraday 

83. Thilorier's Apparatus for liquefying Carbonic Acid 

84. Cold produced by a Jet of Air ..... 

85. Condensation of water vapour by rarefaction of the air 

86. Freezing Water by Evaporation .... 

87. Freezing Water by the Evaporation of Ether 

88. Carre's Freezing Apparatus 

89. Family Ice-machine 

90. Box for collecting solid Carbonic Acid 

91. Crystallization of Alum 

92. Maximum and Minimum Thermometers 



205 
207 
210 
213 
214 

216 
221 
222 
223 
224 
226 
228 
230 

231 
232 
236 

237 
241 

251 



WEIGHTS AND MEASURES, 



In general, throughout the following treatise, the figures of the French 
metric system have been placed in juxtaposition with their English 
equivalents, or the latter have been substituted for them. In some 
instances, this could not be done without an entire reconstruction of 
the author's argument. The following short table of equivalent weights 
and measures may therefore be found useful : — 



I kilogramme 
igo* do. 

I gramme 
loo da. 

I metre 

I decimetre 

I centimetre 

I millimetre- 

I kilometre 

* cubic metre 

I ir decimetre 

I ,, centimetre 

I litre 

I decilitre 

I. centilitre 



2*205 lbs., avoirdupois. 
I cwt. 3 qrs. 24*462 lbs. 
0544 drams avoirdupois. 
3 oz. 8 438 drams avoirdupois, 
39/37°79 inches,. 

3"937079 *r 

0'3937079 r> 
0*03937079,,. 

1000 metres = 1093*63305 yards. 

61027*051 + cubic inches. 

61*02705. + 

0*06112 + ,, „ 

1 cubic decimetre — 1*760 + pints. 

0*1760 + pints. 

0*0704 h pints* 



THE THERMOMETER. 



The range of temperature from the freezing to the boiling point of 
water is divided by Fahrenheit's scale inf.c 180 portions or degrees, 
numbered from 32 to.212 . 

In the Centigrade scale the same range of temperature is divided into 
100 parts, numbered from o to ioo°. 

Hence, to convert degrees of Fahrenheit into Centigrade, subtract 
32, and multiply the remainder by {, and — 

To convert Centigrade into Fahrenheit, multiply the Centigrade by 
f> and add 32 to the product 



THE 

PHENOMENA AND LAWS OF HEAT 



CHAPTER I. 

GENERAL PHENOMENA OF HEAT. 

I. Our ordinary perceptions of Heat distinguished from its 
proper phenomena. 

When we touch any of the bodies that surround us, we 
generally recognise a difference in their state : some com- 
municate to us the sensation of heat, others that of cold. 
This difference is merely relative to our sensations ; it 
depends on the fact that the sense of touch undergoes 
a particular modification, which causes a sensation to be 
experienceu, and a judgment to be formed concerning it, 
practically at the same moment. This special sensibility is 
given to us that we may always be on the watch for our 
preservation. But it is not this kind of action with which 
we are concerned in the present work. We have to inves- 
tigate heat and cold apart from ourselves as exhibited by 
inanimate bodies. We have to discover what phenomena 
occur when such bodies act one upon another ; what modi- 
fications they undergo in becoming hot or cold. And in the 
degree that these modifications impress our senses we shall 
make those impressions also contribute towards the judg- 
ments we have to form. In most cases the sense of sight 
will reveal what is taking place ; but it may happen that our 
hearing or our sense of smell may also be called upon to 

A 



t THE PHENOMENA AND LAWS OF HEAl". 

assist in the observation. It is thus that the human mind 
collects tfre various data furnished by the senses, and makes 
such an orderly arrangement of observed facts as to obtain 
a knowledge of nature. 

The following striking example will teach us to be ever 
on our guard against the sense of touch when heat is in 
question. Plunge the right hand into a vessel of tepid water 
and the left into one of iced water. Then put them both 
into water of the ordinary temperature. The latter will now 
appear to be cold, if we decide according to the sensation 
experienced by the right hand, but warm if we judge by our 
left ; and yet in both cases it is the same water, having the 
same uniform temperature. The different manner in which 
we prepared our hands is the sole reason why we experience 
different sensations. 

The same thing takes place when we pass immediately 
from a hot bath into the open air — the temperature feels 
cold. But if we pass out of a cool room or vault into the 
same atmosphere as before, it will seem to be warm. 

It thus appears that there is no essential difference be- 
tween heat and cold when we abstract our sensations, and 
consider only the body which impresses us. The action of 
heating is simply the inverse of that of cooling, and the 
word heat designates the cause of this kind of action. 

Let us now prepare ourselves to study this subject by 
separating from amongst the thousand phenomena presented 
by nature those which belong to heat. It is certain that 
our first view will be very incomplete ; but as we advance 
we shall become better observers, and at every step a new 
discovery will recompense our toil, and urge us to further 
advances. We may not succeed in seeing and understand- 
ing all that we desire ; but we may, at least, derive profit 
from the study in proportion to our means, and the intelli- 
gence we bring to bear upon them. 

2. Heat produced by the atomic movement which produces 
chemical action. 

We are in the midst of winter ; the surface of the earth is 
white with snow, the rivers are frozen over ; but the room 



GENERAL PHENOMENA OF HEAT. 3 

we inhabit is warmed by a brilliant fire. These conditions 
are admirably well adapted for the commencement of our 
investigations. 

What is a fire ? What, for example, takes place in the 
grate ? We must here have recourse for a moment to the 
science of chemistry ; but at present our object is only to 
obtain from it some two or three very simple ideas, requiring 
a very slight effort of the intellect to comprehend. 

The combustible substance burns in a current of air, 
which enters the room through the interstices of the doors 
and windows, and rises in the chimney after passing over 
the coals. This air becomes modified in the act of support- 
ing combustion. Air is formed of two parts — the one called 
nitrogen, the other oxygen. It is the latter which gives rise 
to the phenomena of combustion whilst uniting itself to the 
carbon; and by this union carbonic acid gas is produced, 
which escapes up the chimney in company with the nitrogen. 
We need not enter into a detailed explanation of this phe- 
nomenon, our object being to concentrate our attention on 
the subject of heat 

The action which takes place in this chemical combina- 
tion may be somewhat understood by imagining oxygen and 
carbon to be formed of particles called atoms, which rush 
together and unite in one body when they are brought 
sufficiently near each other. It is this sudden rush or shock 
of the particles which causes combustion. It produces at 
once heat and light; for the flame that we see is only a phe- 
nomenon which takes place over the coals, and which we 
judge of according to the impression it makes on our eyes. 

Are we then to understand that there can be no chemical 
combination producing heat without the simultaneous pro- 
duction of light? Take a single example from a thousand 
that might be cited to prove the contrary. Powdered 
sulphur is mixed with iron filings, and a hole being made in 
the ground, it is filled with the mixture, which is afterwards 
covered with earth and watered ; in a little time the mass 
heats spontaneously, swells up, raises the earth, and dis- 
engages vapours. This experiment was first made by Nicolas 
Lemery, a chemist of the seventeenth century, who was 



4 THE PHENOMENA AND LAWS OF HEAT. 

educated in a very humble way at Rouen, his native place, 
but lived to acquire an immense reputation as a scientific 
chemist. His object having been to explain volcanic ac- 
tion, this little experiment came to be known as " Lemery's 
Volcano." His explanation of that wonderful natural phe- 
nomenon is not now admitted, but the experiment remains 
as a curious example of chemical combination ; iron and 
sulphur under the influence of water unite to form a solid 
brown body, called sulphide of iron, and at the instant of 
this union there is a production of heat without light. Heat, 
then, is a consequence of the motion of the atoms of matter. 
If we pass its effects in review, we shall always discover in 
them motion as the correlative of heat. 

And first let us inquire into the transference of heat from 
the fire into neighbouring bodies. 

3. Transference of Heat by radiation. 

The heat which warms us, even at a distance, through 
the air interposed between our bodies and the burning 
coal, may be intercepted by a common wooden screen. 
The matter of which wood is composed is a barrier to 
heat, as any opaque body is to light. If, however, we 
substitute for the wooden screen a pane of glass, we shall 
feel the heat at the same time that we see the light from the 
coals through the pane. It is not because the glass gets 
warm, and afterwards acts upon us; for the effect is im- 
mediate, and a considerable time is necessary to make the 
glass hot. When the sun throws its rays on our windows 
the light and heat instantly traverse the glass, whereas both 
are completely stopped by the walls. Now, the sun is an 
immense fire, more than ninety millions of miles distant 
from our globe — so far distant, in fact, that, if we travelled 
at the rate often miles an hour, it would take more than a 
thousand years to reach the sun ; and yet its rays require 
only eight minutes to reach us. To conceive this, imagine 
a wave of motion commencing at the sun, and spreading 
itself step by step across the celestial space, much like 
those familiar circles produced by the splash of a stone on 
the tranquil surface of water : how gradually it extends 



TRANSFERENCE OF HEAT. 5 

farther and farther from its centre, until at last it strikes the 
shore, where it dies out. Now, it is obvious that we can 
follow the liquid wave with the eye, because its speed is 
not too great. We have, then, but to imagine a speed two 
or three million times greater in order to grasp the idea of 
the propagation of the solar rays. Our ordinary fires act 
in the same manner ; they are centres of motion ; they 
resemble little suns of brief duration ; they radiate heat 
around them in obedience to a general law, and every ray 
is the seat of motion. 

4. Transference of Heat by conduction. 

Heat extends itself by still another method. Let the 
extremity of a bar of iron be surrounded by red-hot coals : 
very soon the other end becomes too hot to be held. The 
bar will have been progressively heated by a communication 
of motion from stratum to stratum, from particle to particle. 
This phenomenon has been called conduction. In wood 
this property is scarcely to be recognised ; a stick of wood 
can be very easily held by one end whilst the other end 
is burning : therefore wood is said to be a bad conductor of 
heat, whilst iron is a good conductor. 

Thus the reader has obtained a general notion of two 
kinds of calorific phenomena, radiation and conduction. 
It will now be necessary to fix our attention on the bodies 
themselves which receive heat, in order that we may classify 
its effects. 

5. The combustion of bodies determined by Heat. 

A phosphorus match brought near the fire, without 
touching the coals, will catch light, to use the common 
expression. Hence heat-rays, when brought in contact with 
certain substances, are said to determine their combustion. 

In this case the phosphorus, which covers the end of the 
match, is heated in the open air. The union of oxygen 
with the phosphorus has formed a white substance (phos- 
phoric acid), which disappears in smoke. The pheno- 
menon is analogous to that which occurs in the combus- 
tion of coal. The chemical combination which takes place 



6 THE PHENOMENA AND LAWS OF HEAT. 

is accompanied with the disengagement of heat and light ; 
thus heat causes the sulphur, which the phosphorus covered, 
to burn, so that it also combines with the oxygen of the air 
to form sulphurous acid gas. Finally, this combustion, first 
of the phosphorus and afterwards of the sulphur, deter- 
mines that of the wood of which the match is composed. 

It will be sufficient for our purpose to analyse what hap- 
pens in the case of the phosphorus, as the same reasoning 
applies to the sulphur and the wood. The atoms of phos- 
phorus are united by a force called cohesion, which force 
opposes itself to their combination with the atoms of 
oxygen. The rays of heat which come from the fire put 
these atoms in motion, acting like some actual force capable 
of destroying cohesion, and rending the atoms apart. When 
this happens, the atoms rush to unite themselves with the 
atoms of oxygen, and phosphoric acid results. 

We have thus briefly studied a case of combustion deter- 
mined by heat in concurrence with the air. There are 
other cases of combustion in which air plays no part, and 
which may be effected when the combustible is placed under 
the exhausted receiver of an air-pump. 

Soak a bit of cotton in a mixture of equal weights of 
sulphuric acid and fuming nitric acid ; afterwards wash it 
with abundance of water, and you will obtain gun-cotton ; 
that is to say, cotton combined with a certain quantity of 
oxygen and nitrogen. If brought near a fire, it will instantly 
ignite and disappear, without leaving any visible traces. 
Evidently the atoms of which the gun-cotton was composed 
have been separated from one another by the action of 
heat, and they have recombined in another form as gases, 
which the air has forthwith carried away. If a more com- 
plete chemical explanation is desired, the case may be 
stated as follows : — Gun-cotton is a combination of carbon, 
hydrogen, oxygen, and nitrogen. When the heat of the fire 
has disassociated these four kinds of atoms, they recombine 
in the following order : the carbon and a part of the oxygen 
form carbonic acid gas, the hydrogen and the rest of the 
oxygen form vapour of water, and the nitrogen remains 
free. This is all done so quickly, that gun-cotton can be 



CHANGE OF BULK EFFECTED BY HEAT. 7 

burnt in the hand without causing the sensation of heat; a 
great flame is produced, which instantly disappears, leaving 
no trace if the gun-cotton has been properly prepared. 

The action which takes place in the combination of gun- 
powder is essentially the same. Gunpowder is a mixture of 
saltpetre, charcoal, and sulphur. Decomposed by heat, its 
atoms enter into a fresh arrangement, producing carbonic 
acid gas and nitrogen gas, and leaving a brown compound 
of sulphur and potassium. It is the great bulk of gas 
which, developed in a fire-arm, presses against its sides, 
and drives out the cannon-ball. 

As the facts we have enumerated belong to chemistry, we 
need not dwell upon them any longer. They are sufficient 
to give us a notion of the constitution of bodies, which may 
be defined as assemblages of particles bound together by 
cohesion, and which heat tends to separate, acting like a 
force contrary to cohesion. In the phenomena specially 
treated by physics, the particles are not supposed to change 
their nature; they simply separate from or come nearer to 
each other, or change their respective positions, always in 
themselves remaining identically the same. They are called 
molecules, and they must not be confounded with atoms, 
which may be of different kinds in the same body, and 
which go to form molecules. Thus water is an assemblage 
of like molecules ; each molecule is composed of two atoms 
of hydrogen and one atom of oxygen. Here we only study 
those phenomena in which there is no separation between 
the atoms, and in which the molecules alone act a part. 

6. Change of bulk effected by Heat. 

Take a copper cylinder or ball, and a ring through 
which it can pass freely. Then let the cylinder or ball be 
heated for some time in a fire, and it will be found that it 
can no longer pass through the ring. After being cooled, 
however, it will again pass freely as before ; from which the 
inference is plain — that copper expands by heat and con- 
tracts by cold. Hence one of the physical effects of heat 
is to change the volume of substances. We can form an 
idea of the rationale of this change by imagining the mole* 



8 THE PHENOMENA AND LAWS OF HEAT. 

cules to recede from, or to approach to, one another. It 
follows, also, that they do not really touch each other, a 
certain force keeping them always at a proper distance, in 
the same way that gravitation keeps the earth and othes 
planets at determinate distances from the sun, and uniter 
the innumerable globes which fill the universe ; for each 
star is a sun, which has its attendant planets, and is the 
centre of a movement which follows the plan traced by the 
Creator. Do not forget this general resemblance between 
the world of molecules and the worlds which revolve in the 
starry heavens. A universal law rules over the movements 
and the positions of both. We have not yet discovered 
this law ; but it is a great deal to have already suspected 
its existence, to have included in the same sentiment of 
admiration these two harmonies, differing only in their pro- 
portions : the one presiding over masses infinitely small, 
which we touch • the other over masses which we may call 
by comparison infinitely great, and which are revealed to 
us by instruments which enable us to explore the immea- 
surable profundities of space. 

7. Fusion and solidification. 

Here is a piece of ice, which we place in a glass vessel 
near the fire. It melts, and in the place of the solid ice we 
have soon an equal weight of liquid water. If now we 
expose this same water to the sharp cold which reigns out 
of doors, it will freeze again. This transformation is the 
effect of heat. To melt ice it must be submitted to the 
action of bodies warmer than itself; to freeze water it must 
be submitted to the action of colder bodies. The two 
operations are of the same kind, but both are inverse the 
one to the other. The sun will gradually bring about the 
same phenomena in a field covered with snow. By its 
heating action the white needles which cover the trees, and 
whose marvellous ramifications are so much admired by all 
observers of nature, will be reduced into drops of water, 
which will fall to the earth ; the carpet of snow which 
preserves the soil from the too sudden variations in the 
temperature of the air will diminish in thickness ; and the 



EVAPORATION AND EBULLITION. 9 

water that results will impregnate the earth, and, reani- 
mating the germs contained in it, will restore to it its 
fertility. 

At nightfall, under the clear starlight, the fusing process 
ceases. The force which produced the result has dis- 
appeared \ but more than this — the celestial space is much 
colder than ice. Thus a part of the water melted during 
the day will freeze again ; and several sunny days must suc- 
ceed each other before the snow will completely disappear. 
Life will thus be revived little by little, and the danger 
that would attend a too sudden change will be avoided. 

8. Evaporation, ebullition, and condensation of vapours. 

Fusion, as we have seen, is one of the effects of the action 
of heat upon solid bodies. There is another effect which is 
manifested on some solids, but more especially on liquids. 
During a very sharp cold, a little snow left in the open air 
on a plate may disappear completely without having suffered 
fusion. The same thing happens to camphor if put in a 
large glass bottle. Little crystals are deposited here and 
there on the interior, changing place slowly if the position 
of the bottle be changed. Let us inquire what has hap- 
pened in this case. 

We shall find on investigation that vapours have been 
formed on the surface of the camphor ; their molecules 
have receded from one another, and constitute a true gas, 
which seeks to diffuse itself throughout all parts of the 
bottle. Some of these parts being colder than others, the 
vapours condense at those particular places, and again take 
the solid state by an action inverse to that of vaporization. 
Thus heat has transformed the camphor into vapour, and 
cold has made it once more assume the solid state. It is 
to this property that we owe the odour of camphor. Its 
vapours enter our nostrils and act on the organ of smell ; 
the same thing happens in the case of all odorous solids. 

Liquid substances, however, evaporate most easily. After 
rain the stones and pavements dry very quickly in the sun, 
a phenomenon which evidently cannot be referred to absorp- 
tion. Take a sheet of paper, weigh it, and then moisten 



TO THE PHENOMENA AND LAWS OF HEAT. 

it with water ; the moisture quickly disappears. Weigh it 
again, and it will be found to possess exactly the same 
weight as before. What has become of the water? It 
exists in a state of vapour, mixed with the air by which 
we are surrounded, and is therefore not visible. To see 
the vapour thus disengaged resume the liquid condition, get 
a decanter of cold water fresh from the well or cellar, and 
well corked, so that none may come out Much time will 
not elapse before dew will be seen deposited on the surface 
of the glass. This happens because the bottle of water has 
cooled the air of your chamber, and the vapour of water 
naturally contained in it has been condensed. The atmo- 
sphere which surrounds the terrestrial globe, and w r hich forms 
a gaseous ocean forty miles in depth, evidently contains 
vapour of water, arising from the evaporation of seas, rivers, 
and streams. When the sky is very clear, this water remains 
as an invisible transparent gas ; but various causes here 
and there determine its condensation, and from this arise 
clouds, fogs, rain, hail, and snow, circumstances determining 
which. We shall have to observe all these phenomena, and 
we shall try to discover the harmony of the laws by which 
they are all regulated. 

Let us return to our observations near the fire. We have 
water boiling in an open vessel ; its vapour rises like a little 
cloud, and we can see bubbles detach themselves from the 
inside, rise, and break on the surface of the liquid. This 
again is the reduction of a liquid to the state of vapour ; 
but, instead of being superficial, it is effected at various 
points of the mass. This phenomenon is called ebullition : 
the action of heat is the same in this case as when it causes 
evaporation. It is simply quicker in its action, and is 
spread over a great number of points. 

Thus water has become known to us in the three different 
states of ice, of a liquid, and of vapour or gas. Its mole- 
cules are always identical in themselves, but they are un- 
equally bound together in these three states. In the solid 
there is a strong attraction between the molecules ; in the 
liquid they can roll easily over one another ; and in the 
gaseous condition they recede from one another, pressing 



MECHANICAL EFFECTS OF HEAT. II 

against obstacles. These different conditions maybe pre- 
dicated of a great number of substances. 

9. Mechanical effects of Heat 

The following little experiment will enable us to form an 
idea of the expansive force of vapour. Fig. 1 represents 




Fig. i. — Recoiling iEolypile. 

a copper ball carried on wheels. The ball is hollow ; some 
water has been poured into it through a tubular opening, 
which has been closed hermetically with a cork. The 
tubular opening thus tightly corked has been turned into a 
horizontal position. A small spirit lamp has been placed 
on the carriage under the ball, and being lighted, its flame 
heats the water ; the cork is thrown out to a distance with 
the noise of an explosion, a jet of vapour issues from the 
pipe, and the carriage is driven back. Now to explain what 
has happened. 

The heat of the flame has reduced the water to the state 
of vapour ; the molecules of water press upon all parts of 
the interior of the ball which imprisons them, like an infinite 
number of little springs bent against the interior surface of 
the vessel ; but as yet there can be no motion, because all 
these pressures counteract each other, some tending to push 
the ball in one direction, others to produce a movement 
exactly equal in the opposite direction. The resistance of 
the cork, however, being less than that of the rest of the 
envelope, it is presently overcome, and the pressure it sus- 



12 THE PHENOMENA AND LAWS OF HEAT. 

tained has ceased to counteract the similar pressure exerted 
on the opposite interior surface of the ball. The moment 
this occurs, the backward pressure, so to call it, makes 
the carriage recoil, whilst the forward pressure throws out 
the cork. 

The same phenomenon occurs when a bullet is fired from 
a gun in shooting ; the butt end of the gun rests firmly 
against the shoulder, so as to resist the recoiling force. The 
gases developed in the barrel of the weapon by the com- 
bustion of the powder (of which we have spoken in a 
previous section), act like the vapour in our last experi- 
ment. In both cases there is a mechanical effect produced 
by the action of heat. Therefore heat is capable of de- 
veloping mechanical force, to overcome resistances, and 
set masses in motion. This is the starting-point with all 
machines which are moved by the agency of fire ; they 
work by consuming coal, or any other combustible — that 
is to say, by the expenditure of heat. As very considerable 
progress has been made in this branch of applied science 
during the last few years, it will not be unprofitable to 
bestow a few minutes on its more careful consideration. 
Some fire-machines are moved by the vapour of water, 
others by air ; we shall select one of the latter to establish 
our principle, because the reasoning will be more simple. 

Very remarkable experiments have been made with the 
hot-air engine by M. Hirn, of Colmar, and they have com- 
pletely solved the problem. We have no need to transport 
ourselves to the actual locality of this great establishment, 
where the labours of the engineer keep pace with the pro- 
gress of science. An ingenious air-engine, recently invented 
by M. Laubereau, will serve for an example on a small 
scale. It will be found to deserve our most serious atten- 
tion, as exemplifying a discovery for which we are indebted 
to the labours of many great minds, and which in a very 
few years has produced a revolution in the physical and 
mechanical sciences. The triumphs of human intelligence 
are seldom, if ever, of instantaneous achievement. It is 
only by the exercise of patience, and as a reward of our 
repeated efforts, that we are permitted to lift the veil of 




Fig. 2— Laubereau's Hot Air Engine. 



MACHINES WORKED BY HEAT. 



15 



truth. But what more legitimate subject of pride than the 
success which awaits such endeavours at last ? 

10. A fire-machine spends Heat in effecting work. 

We propose to place our air-machine in a large box, 
entirely surrounded with ice, as by this arrangement a 
machine of very small size will suffice for the experiment. 
In figures 2 and 2 a it is represented in perspective and in 




Fig. 2 a. — Section of the same Engine. 



section. Two vertical copper cylinders are united by a 
pipe. The piston of the smaller one, alternately rising and 
falling, turns the spindle or axletree of the machine. On 
this axle is adjusted the fly-wheel which serves to regulate 
the rotatory movement; and a roller (not represented in the 
engraving), upon which a cord is wound, from which 
depends a weight. The work of the machine, used as a 
model, is to raise this weight ; but in actual business, a 
pulley and strap are provided, by which the motion of the 
machine is transferred to the tool. The work effected by 
the tool is computed as being equivalent to raising a weight 
to a certain height, and it is customary to multiply the 



1 6 THE PHENOMENA AND LAWS OF HEAT. 

weight raised by the height, in order to measure the work, 
For example, a unit of work is the elevation of one kilo- 
gramme to the height of one metre. Ten times this amount 
of work would be to raise ten kilogrammes to the height of 
one metre, or, what would be the same thing, one kilo- 
gramme to the height of ten metres, and so on. Hence a 
kilogrammetre is called the unit of work. 

Let us now see how the action of heat determines the 
movement of the piston. 

The larger of the two cylinders is completely closed. Its 
lower extremity is extended in the form of a bell, so that it 
may be heated with coals placed on a grate, or with any 
other combustible. In the engraving a gas-jet serves to 
heat the cylinder. The upper part has the same form, but 
it is furnished with a double case, so that a current of cold 
water passing between the two cases may prevent this part 
from getting hot. This current of water is formed by the 
simple contrivance of a pump, which the axle of the 
machine sets in motion. The cylinder contains a mass of 
plaster between two bell-shaped pieces of metal which fit 
exactly to the upper and lower extremities, as the whole 
thing is raised or lowered. To effect this displacement a 
handle and connecting-rod is adjusted to the axle, and acts 
upon the plaster after traversing the double cases through a 
box stuffed with tow. The position of this handle in rela- 
tion to that of the piston has been so calculated, that the 
mass of plaster may be in the upper or lower part just at 
the right moment. 

We have thus a bulk of air confined in the two cylinders, 
the pressure of which upon the piston is exerted upwards, 
whilst that of the atmosphere is exerted downwards. It is 
understood that the piston rises when interior pressure is 
greater than the exterior pressure, and falls when it is less. 
It is the action of the fire and the current of cold water 
which effects these changes of pressure. To obtain a per- 
fect conception of this action, suppose, to begin with, that 
the mass of plaster is in contact with the refrigerator ; the 
heat of the fire warms the air of the machine, augments its 
elastic force, and the piston mounts. When it has arrived 



MACHINES WORKED BY HEAT. 1 7 

at the proper point in its course, a second period com- 
mences ; the plaster begins to approach the heated portion, 
and as it conducts heat badly, and therefore does not allow 
it to pass by it, the furnace ceases to act on the air; this 
cools by giving up its heat to the cold water, its elastic force 
diminishes, and the piston descends, partly in conse- 
quence of the impulsion transmitted to it by the fly-wheel, 
and partly because the atmospheric pressure is now greater 
than that of the air contained in the machine. Then the 
plaster again rises ; the heat of the fire acts freely on the 
air, re-heats it, and the motion continues by the simple dis- 
placement of the mass of plaster, which acts somewhat in 
the manner of a screen, alternately intercepting the action 
of the fire and that of the refrigerator, and the bell-like 
form of which serves to isolate the interior air as much as 
possible — first from contact with the cold surface, then from 
that of the hot surface. The mechanism being understood, 
we shall now proceed to our experiment. 

This little machine, with its furnace lighted ready for 
action, is placed in the box surrounded with ice, with which 
the reader will remember we were prepared at the beginning. 
The charcoal has been weighed, and we secure the dryness 
of the ice by opening a stopcock, through which any water 
produced by fusion may run out. Openings conveniently 
disposed permit the entrance of air sufficient to make the 
charcoal burn, and the gases developed by the combustion 
escape by the chimney, after having traversed a serpentine 
tube plunged in the ice, in order to be thoroughly cooled. 
The machine is put in at rest, and while this is so the fire 
has no other effect than to melt the ice. At the end of a 
certain time, we ascertain on the one hand the weight of 
.he remaining charcoal, and, by subtracting it from that we 
obtained before the experiment commenced, we find the 
weight of charcoal consumed, and this number serves to 
measure the quantity of heat developed by the combustion. 
On the other hand, we collect the water which results from 
the melting of the ice, and ascertain its weight, which is 
evidently proportional to the quantity of heat developed, 
and, in consequence, to the weight of charcoal consumed ; 

B 



1 8 THE PHENOMENA AND LAWS OF HEAT. 

so that if, in another experiment similar to the preceding 
one, we made the operation last so much longer that the ice 
melted might be double, we should find the charcoal burnt 
also double. It appears, from numerous experiments of this 
kind, that a given weight of charcoal may, in burning, be 
made to melt a quantity of ice about a hundred times 
greater than itself. 

After this first observation, set the machine in motion, 
and measure, as before, the weight of charcoal consumed 
and of ice melted ; add to this the height to which the 
weight is raised by the action of the machine, and also the 
value of this weight. For simplicity's sake, suppose that 
the weight of charcoal consumed be the same as before, 
shall we Lave the same amount of ice melted ? 

Only a very few years ago a savant to whom this question 
had been put would, very probably, have answered, Yes, 
because the quantity of heat developed by the combustion 
of a given quantity of charcoal is always the same, whether 
the machine be at rest or in motion, and nobody thought, 
at that time, there could be any intimate relation between 
heat and motion. Now we are much better informed, and 
we know that the quantity of ice melted during the com- 
bustion of a given quantity of charcoal is less when the 
machine works than when it is at rest. If the work effected 
was to raise i kilogramme to the height of about 400 
metres, the amount of ice melted would be about 12 
grammes less; if the work was to raise 2 kilogrammes 
to the same height, the amount would be 24 grammes less : 
thus the loss is known to be proportional to the work done 
by the machine. 

From these two comparative observations it is concluded 
that heat may disappear or be expended at the same time 
that a certain amount of mechanical work is effected ; that 
is to say, a certain measure of resistance is overcome by the 
material system in which this disappearance takes place. 
We must, therefore, add to our list of the phenomena of 
heat the mechanical effects produced by it. We have seen 
it employed in changing the volume of bodies, afterwards 
infusing solids and vaporizing liquids ; further, it can be 



ANIMAL HEAT AND MECHANICAL WORK. 1 9 

employed as a motive power. In the latter case none of 
the preceding effects will be observed to reveal its presence. 
When these effects only are taken into account, we say thai 
the heat employed to set the mechanism working has dis- 
appeared; but we know that nothing in reality can be 
annihilated. Mere matter is incapable of moving itself; it 
can only obey the laws which reign in the universe. The 
elevation of a weight effected in our experiment is a motion 
equivalent to another motion which we call heat, which is 
hidden from our sight, but which we can conceive by calling 
our intelligence to the assistance of our senses. The task 
is a long and difficult one, and, before seeking to trace 
effects to their causes, it is necessary to know those effects 
accurately. To place them clearly before the reader is the 
object of this work. 

After the experiment we have just described, the problem 
which follows cannot be difficult to solve, and it will open 
out to our contemplation a new horizon. 

1 1. Relation between animal Heat and mechanical work. 

A man is placed in a box in such a manner that the 
quantity of natural heat disengaged from his body, and the 
quantity of oxygen he takes in from the air for respiration, 
may both be measured. Question — Will he disengage the 
same quantity of heat for the same quantity of oxygen 
consumed if he remain at rest, as if he does some me- 
chanical work, such as raising a weight by a rope passing 
over a roller ? 

The reader is of course aware that respiration is a 
function of our organization designed to effect the intro- 
duction of atmospheric oxygen into the blood. This oxygen 
causes a slow combustion in our bodies, and in consequence 
an incessant production of heat. The quantity of heat 
really brought into play may be measured ■ by the data 
afforded by the amount of oxygen consumed. Now, if a 
certain amount of mechanical work is effected, a part of the 
heat employed is definitively expended for this effect, and 
cannot serve to heat othei bodies. This is proved by the 
fact that we find a deficit proportional to the work effected, 



20 THE PHENOMENA AND LAWS OF HEAT. 

when we compare the quantity of heat employed to heat 
the surrounding substances and the quantity which the body 
has furnished. Thus a man will act like a fire-machine, 
and the result of the experiment will be the same in both 
cases. Whilst consuming the same quantity of oxygen, he 
will disengage less heat if at work than if he be at rest. 

This experiment has been tried by M. Hirn upon himself 
and upon several individuals of different ages and of both 
sexes : all the data he has thus obtained conspire to the 
same result. 

It is necessary to guard ourselves against the supposition 
that the disappearance of heat occasioned by mechanical 
work is necessarily accompanied by a cooling of the body. 
A little brisk work in general warms the body, and in this 
state it disengages more heat than when in a state of repose. 
But it also consumes more oxygen, so that our principle 
remains unaffected by this well-known fact. It is for this 
reason necessary that the man who does manual labour 
should have a greater volume of air to respire than the 
student. The workshop must therefore be commodious and 
well ventilated, care being taken that there are never too 
many individuals assembled in a given space, if we wish them 
to work satisfactorily, and preserve their health. And let 
us add further that their food should be good and abundant, 
so that they may be in the right condition to effect work. In 
fact, as the oxygen of the air serves as the agent of combustion 
in our blood, so the food we digest furnishes the combustible 
elements. If insufficient, the air becomes useless : the blood 
which circulates in all parts of the body will take the sub- 
stance itself of the tissues, and draw it into contact with the 
oxygen, which burns it. The body then becomes lean, as it 
is principally formed of carbon and hydrogen ; and as these 
two substances constitute, with oxygen, carbonic acid gas 
and vapour of water, it may be said that by the w r ant of 
food the body of an animal slowly consumes itself, so as to 
leave what may be called a little cinder only. The foods 
specially adapted to produce animal heat are saccharine 
and spirituous matters, because carbon and hydrogen pre- 
dominate in them. This is the reason why men who work 



ANIMAL HEAT AND MECHANICAL WORK. 21 

hard find support from alcoholic liquors. It is surely 
unnecessary to add that such means should be resorted 
to wisely and temperately ; the bad effect produced on the 
organization by the abuse, to which workmen are so often 
tempted, destroys all the good that would arise from their 
use in moderation. 

By these considerations, we are led to regard men and 
animals as machines, which effect work with the aid of their 
muscles, by spending heat, exactly like fire-machines. The 
will determines the motion, and maintains it, provided that 
the machine is able to consume in its muscles as much heat 
as is proportional to the work effected. Here, however, we 
are constrained to admire the superiority of living beings 
over machines. When a steam-engine or an air-engine 
works, the whole of the heat which is developed in its 
furnace cannot be employed with mechanical effect : a 
considerable part serves to warm the neighbouring bodies. 
In fact, this lost heat is nearly twenty times the amount of 
that which we estimate as being effective. Suppose, for 
example, the mechanical work effected by such a machine 
to consist in the elevation of 65 kil. to the height of 4,800 
metres ; 1,835 grammes of carbon must be burned under the 
boiler. The weight we have chosen is that of an ordinary 
man, and the height, that of Mont Blanc. When a tra- 
veller ascends this mountain, he raises his own weight, and 
effects with his muscular power the work we have just 
estimated. Starting from Chamounix in the morning, he 
would pass the night at the Grands Mulets, start again next 
morning, and arrive at the summit towards evening : he is 
at Chamounix on the return journey towards the fall of 
night. The excursion lasts about 28 hours ; but the actual 
walking, consequently the production of work, lasts only 
17 hours. According to M. Hirn, a robust man who 
ascends Mont Blanc thus consumes 132 grammes of 
oxygen per hour, which amounts to 2,244 grammes in 17 
hours. The combustion taking place in his body is equal 
to that of 833 grammes of charcoal. Comparing this 
quantity with that consumed by our steam-engine, it will 
be seen that it is less than the half. 



22 



THE PHENOMENA AND LAWS OF HEAT. 



12. Electricity produced by Heat. 

The part played by heat does not terminate here. It is 
capable of producing electricity under its two principal 
forms. In the seventeenth century, travellers brought from 
the isle of Ceylon some little green stones, in the form of 
prismatic needles, which, by heating, acquired the property 
of attracting light substances. The natives of the country 
called these stones Toumamal (ash-drawers), because, 
when placed on hot ashes, they attracted them. This 

name has been corrupted into 
Tourmaline. We now obtain tour- 
malines from various localities ; those 
from Brazil, which are green or blue, 
are the best for experimental pur- 
poses. Fig. 3 shows the arrange- 
ment adopted by the celebrated 
Hatiy. The tourmaline being made 
hot, is placed on a little support, 
balanced with two metallic bails, 
so that it may remain horizontal 
when set on a point. If now a 
piece of rod glass, well rubbed, is 
presented to the extremities of the 
stone, it will be observed that one 
extremity is attracted, whilst the 
other is repelled ; an effect which is 
attributed to electricity. The tour- 
maline, therefore, becomes electrical 
by warming. 
Since 182 1, another method of producing electricity by 
heat has been known. It is under the form of a current, 
and this property of heat is often utilized for the purpose 
of studying some of its properties, particularly radiation. 
Prepare some little bars of two different metals, such as 
iron and copper, and solder them alternately by their ex- 
tremities in such a way that each joint shall be between 
iron and copper, and the paired joints be all kept in one 
line, as fig. 4. Unite the extremities of this simple con- 




Fig. 3. — Haiiy's Tourmaline 
Apparatus. 



ELECTRICITY PRODUCED BY HEAT. 



*3 



struction by a copper wire, and you will have constructed 
an instrument called a thermo-electric pile (fig. 4.) To make 



/^r 



c * yv^— g ^rm 



rrr^ 




Fig. 4.— Thermo-electric Pile. 




Fig. 5.— Pile and Galvanometer. 



24 THE PHENOMENA AND LAWS OF HEAT. 

it active, it is only necessary to heat the joints of one side^ 
leaving the others cold. The copper wire then assumes a 
peculiar electric state ; a current is established. One of its 
effects is to cause the magnetic needle to deviate from its 
natural direction when brought near it. This effect may be 
rendered very apparent by suspending the magnetic needle 
very delicately, and arranging the wire in a proper manner ; 
an instrument thus constructed is called a galvanometer. It 
is sufficient to touch with the finger one of the joints of the 
pile, to make the magnetic needle deviate very greatly. 
Fig. 5 shows the pile and galvanometer arranged for an 
experiment. 

We need not occupy ourselves further with this class of 
phenomena, as it belongs more appropriately to electricity. 
The subject has been briefly alluded to, for the sake of con- 
sidering heat under all its aspects. We cannot dismiss it, 
however, without referring to a conclusion resulting from 
these observations, which is well calculated to impress the 
imagination. The terrestrial globe is an assemblage of 
different substances, connected or soldered together in some 
fashion, if one may use the expression. In executing its 
daily revolution on its axis, this globe successively presents 
to the sun different portions of its surface, and these parts, 
being heated by the rays of the sun, develop electricity. 
The electrical action circulates thus every day round the 
earth, and the result is an immense current, capable of pro- 
ducing such permanent effects as the direction of the 
compass. Thus we begin to see how great is the part 
played by heat in the universe, and how motion and life on 
the earth are closely allied to solar action. 

13. Fire-worshippers. 

The sun is the apparent source of joy, of fertility, and of 
the life inherent in all nature. Is it wonderful that man, 
contemplating the active power of this luminary, has made 
it so often the object of his worship? Impressed by a 
phenomenon which could not but command his wondering 
admiration, he has rendered to the most brilliant and 



FIRE WORSHIPPERS. f { 

beneficent of the Creator's works that homage which was 
due to its Author. Such has been the commencement of 
religion probably in all nations, for constantly, amid absurd 
fables born of ignorance, one always meets with the worship 
of the sun, or of fire, its image. In places so widely separ- 
ated by time and space as ancient Persia and Mexico, 
this has been the origin of certain customs and ceremonies 
of which we need not more particularly speak save to 
remark on their sanguinary character in the latter country. 
In Peru, the system of superstition upon which the Incas 
founded their authority was unique. Manco-Capac, the 
legislator of the Peruvians, taking advantage of the venera- 
tion in which the sun was held, pretended that he and his 
wife were children of this star. He was listened to and 
believed, and became the first Inca; his descendants were 
called the children of the sun, and they alone were deemed 
capable of reigning. Their religious worship was directed 
to the objects of nature, to the contemplation of the order 
and beneficence displayed in the universe ; the ceremonies 
were mild and human; they offered to the sun the sub- 
stances which his heat had caused the earth to produce, a 
few animals which they used as food, and certain works of 
art. As the character of their institutions was mild and 
beneficent, so civilization was more advanced among these 
people than among the other races of America. Agricul- 
ture was held in such honour, that the " children of the 
sun " cultivated a field with their own hands, calling this 
duty their triumph over the earth. 



CHAPTER II. 

ON THE EXPERIMENTAL METHOD AND THE THERMOMETER. 

i. Aim of physics. 

There are two methods of arriving at a knowledge of 
the laws of heat, — the one used by philosophers, the other 
by physicists. The former rely on metaphysical considera- 
tions—on certain general ideas relative to the constitution 
of the universe ; and, having laid down a series of rational 
principles, they proceed to predicate the phenomena which 
ought to result from them, reasoning from cause to effect. 
The physicist, on the contrary, first applies his attention to 
the exact observation of phenomena; he will measure, he 
will count, he will weigh what he has before him, so as to 
become well acquainted with its various numerical relations ; 
he will conceive experiments in which the quantities to be 
measured are plainly separated ; he will invent instruments 
with which he will effect exact measures ; and, when he 
shall have worked out the result of his researches with the 
precision of a mathematical statement, he will feel satisfied 
that he has found the laws of the phenomena, — in other 
words, the method of nature. 

For example : a leaden ball suspended by a long string 
to a fixed support naturally oscillates when it is let go, and 
we can easily count the number of oscillations it makes in 
a minute. Now shorten the string, reducing its length to a 
quarter of what it was ; count the oscillations again : we 
shall find that they are double in number during the same 
time. If the length be again reduced to a ninth part, the 



EXPERIMENTAL METHOD — THE THERMOMETER. 27 

number of oscillations will be found tripled. The phe- 
nomena may be represented by a mathematical statement^ 
whatever be the lengths compared : this statement expresses 
the law of oscillations. 

When by following this method a great number of laws 
have been obtained relating to phenomena evidently due to 
the same cause, we may try to trace the effects to their 
causes ; but in doing this we leave the true scientific path, 
and the conceptions to which we attain are always uncertain. 
The degree of probability to which they may lay claim 
depends on the number and exactitude of the observations 
which led up to them ; they are called hypotheses, and are 
not necessary for the attainment of a knowledge of nature. 

In glancing over the history of the sciences, we shall find 
that the experimental method is of recent origin. The 
ancients worked metaphysically : Aristotle is the most cele- 
brated master in this school. His doctrine is based on a 
crowd of nominal distinctions, in which are considered the 
qualities of things only, and not their quantities. Imagina- 
tion effected everything. And yet it was Aristotle's logic 
which prevailed in the schools so late as the sixteenth 
century. The celebrated Descartes, who lived from 1596 
to 1650, himself contributed to retard the progress of the 
natural method. " Before seeking the laws of weight," said 
he, " I must learn what weight is; because causes ought to 
explain effects." So far was he from being in the track 
which has led to so many surprising and useful discoveries 
in recent times. 

Galileo, who taught publicly in Florence about this time, 
should be looked upon as the founder of the natural 
method. He 'has given its theory, and he has put it in 
practice, inventing the instruments and conceiving the 
experiments. " We cannot," said he, " know the essence 
of things : the absolute escapes us ; we can only appreciate 
the relative. The causes are of little consequence ; it is 
the necessary relations or laws of things which must be 
discovered." And he discovered the rotation of the earth, 
the oscillation of the pendulum, the weight of the air, the 
fall of bodies ; he created mechanics, or physics, properly 



l8 THE PHENOMENA AND LAWS OF HEAT. 

so called. He was a mathematician, and he knew how to 
read in the book of nature — a book written, as he said, in 
mathematical language. And yet it must not be supposed 
that all his science was hidden in formulae accessible only to 
a small number of adepts. Galileo had the great merit of 
rendering science popular. He once wrote to a friend, — 
" I have remarked that many of the young men who fre- 
quent our universities to prepare themselves for the liberal 
professions, often show very little taste and aptitude for 
the study of natural philosophy. Others, on the contrary, 
whose brains are more happily constituted in this respect, 
give themselves up to industrial or domestic occupations, 
without inclining to philosophize, because they imagine 
philosophy is contained in great books written in os and us, 
which they are not able to re-ad. Ah ! how I wish they 
knew that the same bountiful Nature has given eyes to them, 
as well as to the readers of Greek and Latin, to behold her 
works, and brains to know and comprehend them/' 

Let us be disciples of Galileo ; let us properly observe 
and analyze, and thus learn to distinguish the law of phe- 
nomena from the hypothesis which pretends to reveal its 
cause. For us to explain a phenomenon is to describe it, 
showing the relation which exists between its various parts, 
or between it and other phenomena already understood. 
We shall be sufficiently acquainted with heat, when, having 
become habituated to grasp a natural tie between its several 
effects, we are able to say which of them will be produced 
under such and such circumstances. 

As to the intimate nature of heat, we are reduced to 
simple conjectures ; and, when one reviews the opinions 
which have been expressed on this subject, it is easy to see 
what errors their authors have been led into for want of 
exact observation. 

2. Hypotheses as to the nature of Heat. 

Until the commencement of the present century the most 
universally accepted hypothesis was that of the materiality 
of heat. According to this hypothesis, heat was a kind of 
fluid matter without weight, different from that which con- 



HYPOTHESES CONCERNING HEAT. 20 

stitutes the molecules of bodies interposed between them, 
and capable of passing from one body into another with 
great quickness ; it was called caloric. A body being 
warmed was supposed to receive from without a certain 
quantity of caloric, which added itself to the caloric already 
contained in the body; when cooled, the caloric, on the 
contrary, was supposed to be given out. 

In combustion the different substances were supposed to 
combine whilst disengaging caloric, because their molecules, 
having changed position, constituted a new body incapable 
of containing the sum of the quantity of caloric contained 
in the substances previous to combustion. The quantity 
of fluid contained in the unity of weight of a given body 
was hence called its spexific heat. For example, carbon in 
combining with oxygen disengaged heat, because the specific 
heat of carbonic acid gas thus formed is less than the sum 
of the original quantities of heat contained in the carbon 
and oxygen which constitute the unity of weight of carbonic 
acid. A few simple observations will prove the error of 
this hypothesis. 

When a piece of copper is filed, heat is disengaged ; 
therefore, according to the preceding theory, the filings 
should possess a specific heat less than compact copper. 
But this is not so. Copper filings, when submitted to the 
action of heat, behave precisely like copper in the lump. 

Take another example : If two pieces of ice be rubbed 
together, and every imaginable precaution be taken against 
warming them by contact with other substances, the ice 
will melt as if put near the fire. The partisans of the 
material theory of heat will say that the specific heat of 
water is less than that of ice, and, in consequence, friction 
determines the escape of heat. The water can no longer 
maintain the solid state, but becomes liquid. Without 
troubling ourselves with the assumption that heat escapes 
through friction, the hypothesis is confuted by the fact that 
the specific heat of ice is less than that of water, contrary 
to the foregoing argument. 

In the first chapter we have described an experiment in 
which heat disappeared, and yet was not to be found again 



30 THE PHENOMENA AND LAWS OF HEAT. 

in the various bodies in play ; such a result could not be 
conceived if it be admitted that heat is a kind of matter. 

This hypothesis is now rejected because a great number 
of facts have been discovered with which it is incompatible ; 
but, as it has for a long time reigned in science, many ex- 
pressions and forms of reasoning are still found which carry 
its mark : so that one must not attribute to them the 
meaning that they had when first brought into use. 

A second hypothesis regards heat as a motion of the 
molecules of bodies, which is accelerated during warming, 
and retarded during cooling, which moreover can be trans- 
mitted from one body to another in the same manner that 
the agitation excited at one point of a mass of water 
is progressively communicated to the whole body by a 
sort of radiation in every direction. The molecules are 
supposed to be distributed under the dominion of a uni- 
versal attractive force, in the midst of a very elastic fluid 
called ether, which is spread through all space ; and it is 
through the mediation of this fluid that the radiation of 
heat and light takes place. Thus, when two bodies are 
brought together, the motion of their molecules tends to 
distribute itself equally, and the effects of heat would be 
due to the reciprocal transmission of these motions. Fric- 
tion develops heat, because, according to this theory, a 
motion is communicated between the rubbing body and the 
insensible molecules of the body rubbed. This motion 
escapes our notice, as do the molecules themselves, in con- 
sequence of its extreme minuteness ; but our senses are 
impressed by the various effects of this motion which we 
call heat It must be remarked here that the one word 
heat is employed in two different senses. Here we speak 
of the effect ; elsewhere the same word may designate the 
cause. A little attention will always distinguish which 
signification must be accepted under given circumstances. 

The disappearance of heat would simply mean the diminu- 
tion of the motion of the molecules, this being transmitted, 
not to the molecules of the neighbouring bodies, but to 
their appreciable masses, which are always put in motion 
when this disappearance takes place. The molecular motion, 



HYPOTHESES CONCERNING HEAT. 31 

therefore, would be converted into the motion of the mass ; 
the heat would be transformed into mechanical work, the 
same as in the inverse case ; in friction and in other circum- 
stances the motion of the mass would be converted into 
molecular motions, the mechanical work transformed into 
heat. 

This hypothesis, which is called dynamic, has been adopted 
by a great number of philosophers since the seventeenth 
century. Indications of it are to be found in the writings 
of Descartes, Bacon, Euler, and others ; but without the 
precision which it has since attained. It was not until after 
the time of Galileo, when physics was really founded on 
experiment, that the metaphysical conceptions took a deter- 
minate form, and became capable of serving usefully as the 
basis of a theory. But in fact it is not sufficient that this 
hypothesis should be definitely traced out that heat should 
be spoken of as a motion ; it is still necessary to say what 
kind of motion is imagined, and what are its laws. Until 
this is done, only the preparation or rough draft of a theory 
is made out. 

Several modern authors have essayed to attain this end, 
and have presented a theory of heat founded on the dynamic 
hypothesis. Imagining a peculiar constitution of bodies, 
they have established formulae representing the laws which 
they supposed to regulate the motion of the molecules, and 
from these they have drawn all the mathematical conclusions 
possible. To make the theory acceptable, it is necessary 
that all these conclusions be confirmed by experiment. If 
a single one is in opposition to the facts, the whole theory 
must be built up again ; the edifice so arduously constructed 
must be destroyed ; and, after having corrected the starting- 
point, the concatenation of conclusions must be recom- 
menced in order to submit it to a new proof. 

Such a labour is a speculation of the mind ; certain men 
of rare genius give themselves up to this, to satisfy their 
insatiable thirst for knowledge ; they contribute largely to 
the progress of physics by enlarging the field for discovery. 
In fact, very often a mathematical conclusion leads to the 
conjecture of a phenomenon until then unknown. The 



3* THE PHENOMENA AND LAWS OF HEAT. 

physicist applies himself to the task ; he collects the ma- 
terials for the new experiment ; he observes, and, if the 
phenomena appear such as he was forewarned, the theory 
is confirmed. However this may be, a theory established 
in this manner is always unstable, like every other pro- 
duction of the imagination. Let an adverse experiment 
be made, and it is overthrown. Newton's theory of light 
has been destroyed by the disciples of Galileo, and that of 
Fresnel, conformable to the results of experiment, has re- 
placed it. Only the law of phenomena established by 
careful observation can rest immutable. This is the secret 
stolen from nature ; it is the victory of our intelligence, 
aided by our senses ; to it we have devoted all the strength 
with which the Creator has endowed us. 

After these reflections, it is hardly necessary to explain 
that we propose to consider only the principal effects of heat, 
to co-ordinate them, and find their laws, without troubling 
ourselves about causes. To accomplish this completely we 
require some knowledge of geometry. But, as we must not 
suppose every one to be acquainted with the elements of 
mathematics, and as this book is intended for all, we shall 
be able to give only a simple sketch, or sort of initiation. 

3. Invention of the thermometer. 

Our first requirement is an instrument to measure the 
effects of heat. That which physicists have adopted is 
called a thermometer. It has many forms, but its essential 
character is to indicate by the changing volume of the 
substance which constitutes it, whether the space it occu- 
pies may be warming or cooling. 

The first thermometer appears to have been constructed 
by Galileo in 1597. It depended on the dilatability of the 
air. A glass bulb was joined to the extremity of a glass 
tube, and a little column of liquid introduced into the tube 
by the open extremity (A, fig. 6). By this simple means, 
a small volume of air was imprisoned in the instrument, and 
this air being separated from the atmosphere by the little 
column of liquid, its quantity always remained the same. 
When an instrument thus constructed is put in a warm 



AIR AND SPIRIT THERMOMETERS. 



33 



place, the liquid index increases its distance from the bulb, 
which shows that the air contained gets warmer, and 
augments in volume, retaining a pressure nearly equal to 
that of the atmosphere, which acts on the index by the 
open extremity of the tube. 

Inversely, when the thermometer is put in a colder place, 
the column of water sinks — that is to say, the index ap- 
proaches nearer to the bulb, because the contained air is 
cooled and reduced in volume. 

Many savans in Galileo's time 
applied themselves to the study of 
the thermometer. In fact, the founda- 
tion of modern physics was then laid. 
They felt the necessity of creating 
instruments for observing natural 
phenomena in a better manner than 
had been yet done. The thermo- 
meter of Cornelius Drebbel, son of 
a Dutch peasant, came rapidly into 
use in Flanders and England. This 
also consisted of a glass bulb and 
tube (B, fig. 6), containing air ; the 
tube is placed vertically, and its open 
extremity plunged under some liquid 
contained in a little dish. To re- 
gulate the quantity of air which 
should remain in the bulb, it is 
warmed, which causes several bub- 
bles of air to escape ; after this, it 

is allowed to cool. The liquid then rises in the tube, and its 
level serves as an index, like that of Galileo's thermometer. 

The first spirit thermometer was constructed in the 
Florentine Academy. The reservoir has the same form as 
above; it is still a glass bulb or cylinder, joined to the 
extremity of a very narrow glass tube ; but it contains spirit 
instead of air. To replace the air naturally contained in it 
by this liquid, an artifice is necessary. The bulb is held 
over a fire whilst the end is plunged under the spirit. The air 
inclosed in the bulb dilates, and bubbles through the liquid 

c 




Fig. 6. — Air Thermometers. 



34 THE PHENOMENA AND LAWS OF HEAT. 

whilst it (the air) is warmed. The source of heat is removed ; 
when the air contained in the instrument cools, and con- 
tracts, and the liquid rises into the bulb, which it nearly 
fills. To remove the remainder of the air contained in the 
bulb, it is again put over the fire, so as to make the alcohol 
boil. Its vapour mixes with the air, and, rushing out in a jet, 
they escape together. At this moment the open extremity 
of the tube is quickly plunged into the alcohol, and the 
heating discontinued. In the process of cooling, the alco- 
holic vapour which fills the bulb condenses, ultimately 
occupying but a very small space ; so that the pressure of 
the atmosphere causes the liquid contained in the dish to 
rise quickly into the bulb. This time it will be entirely 
filled. Yet there is always a small bubble 
of air which it is necessary to remove. To 
ft effect this, before removing the dish of alcohol, 

the instrument must be allowed to cool 
thoroughly; a string is then attached to the 
tube, and it is swung rapidly with the bulb out- 
wards, like a swing. By this motion the heaviest 
particles remove themselves farthest from the 
centre ; the alcohol contained in the tube enters 
the bulb, and the bubble of air contained in 
the bulb approaches the operator's end, and 
consequently can be completely removed from 
the liquid. When this is completed, the instru- 
ment is in the state represented by fig. 7. To 
regulate the quantity of alcohol which should 
remain, the bulb must be warmed a little ; the 
alcohol then expands, and several drops escape 
by the opening. It is allowed to cool; the 
alcohol contracts, and its level will be found 
AiShoi nearer the bulb than before. This manoeuvre is 
Thermometer, repeated, if necessary, until its level is found to 
be about a third of the length of the tube, 
measuring from the bulb. 

To finish the construction of the thermometer, it is only 
necessary to direct a blow-pipe flame on the end of the 
tube ; the glass softens, and the opening can be closed by 




MERCURIAL THERMOMETER. 35 

drawing it out. There now remains some air confined 
above the level, having no communication with the atmos- 
phere. When the bulb is warmed, the alcohol expands, its 
surface presses on this air, compresses it, and augments 
its elastic force ; this is the reason that the alcohol does not 
vaporize, and that it can be heated somewhat considerably 
without being seen to boil. An explanation of this phe- 
nomenon will be given further on (Chapter VIII). Ordi- 
narily the alcohol is coloured red, that the movements of its 
level may be more readily followed. 

The spirit thermometer offers several essential advantages 
over those of Galileo and Drebbel ; as, owing to its being 
shut, the atmosphere has no effect on the bulk of the 
alcohol ; whilst in the other two, which are open, the 
atmosphere exerts pressure on the index, which varies as 
its pressure changes while there may be neither warming 
nor cooling. What we have to observe on the whole is, 
that the displacement of the index in these two instruments 
is produced by heat and by atmospheric pressure, and that 
it is difficult to discern the specific effect due to each of 
these causes. 

About 1680, mercury began to be used in place of alcohol, 
and it was found to possess great advantages. Mercury may 
be made hot or cold quicker than alcohol : as it is opaque, 
it may be very easily seen in the tube, even when this has 
an interior diameter excessively small, which condition 
renders the instrument more sensitive. It is also easier to 
obtain mercury perfectly pure. In fine, mercury may be 
heated to a much greater degree than alcohol, without being 
transformed into vapour. 

A mercurial thermometer may be constructed in the same 
manner as the spirit instrument, except that there is no 
bubble of air to remove by the swinging motion, and it is 
not necessary to allow the tube to remain full of air. When 
the necessary quantity of mercury has tfeen introduced into 
the instrument, so that its level may be about a third of the 
whole length, reckoning from the bulb, the latter is heated 
until the mercury reaches the opening, which is then imme- 
diately closed with the blow-pipe. By this means air is 



3& THE PHENOMENA AND LAWS OF HEAT. 

altogether excluded ; and when by cooling the level has 
again nearly reached the bulb, there is a perfect vacuum 
in the upper portion of the tube. 

So far as we have just constructed it, the thermometer 
will perfectly indicate, by the displacement of its level, 
whether there be a warming or cooling action going on; 
but as yet it does not exactly measure the effect of the heat. 
To do this it must now be graduated. 

4. Graduation and use of the thermometer. 

It is since the year 1741 that the plan for graduating 
thermometers adopted by the Swedish physicist, Celsius, 
has been followed. Until then, there had been no fixed 
plan, and the numbers indicated by various thermometers 
did not accord together. 

On the whole length of the tube of the thermometer a 
scale of equal divisions is marked ; each division is called 
a therm ometric degree, and these degrees are marked by 
consecutive numbers. This graduation must be made in 
such a manner that all the thermometers placed in the same 
circumstances, — for instance, in the same quantity of water, 
— have their levels stationary, after some time, before the 
same number on the scale. The plan of Celsius permits the 
realization of this condition, so that any number of thermo- 
meters may be compared together. 

According to this plan, the thermometer is allowed to 
remain immersed in ice, placed in a vessel pierced full 
of holes, so that the water resulting by liquefaction may 
run out ; when this precaution is taken, the level of the 
mercury will become stationary. Its place is then marked 
on the tube. The thermometer is then immersed in the 
vapour produced by water in full ebullition (fig. 8). The 
level rises by the expansion of the mercury, and soon 
becomes stationary. Its place is again marked on the tube. 
Two fixed points are thus obtained, which serve to regulate 
the scale. 

The thermometer being adjusted, for example, on a 
small piece of wood, two marks are made opposite the 



GRADUATION OF THE THERMOMETER. 



37 



fixed points, and the figures o and too are written by 
their side.* The intervening space is then divided into 
one hundred equal parts, the division being extended to the 








FlC. 8. — Apparatus for graduating 
the Thermometer. 



Fig. 9. — Ordinary 
Thermometers. 



extremities of the tube. The numbers are the same on 
either side of zero, as indicated by fig. 9 ; and to distin- 
guish them, it is only necessary to say that they are above or 
below zero. This graduation is called' the Centigrade, and 
it is constantly spoken of under that name. 

The word temperature is adopted to designate the state of 



* The graduation here given is that of the Centigrade thermometer. 
The principle of construction is, of course, the same fox any other scale 
in use. 



38 



THE PHENOMENA AND LAWS OF HEAT. 



bodies relatively to heat. When a body is placed in contact 
with a thermometer, the mercury rises or falls in the tube 
according as the body is hotter or colder than the thermo- 
meter, and its level stops at some division of the scale. 
The corresponding number represents the temperature of 
the body. 

The thermometer thus serves to indicate the calorific state 
of bodies. If its level is stationary, the temperature is 
constant ; if it rises or falls, the number of degrees through 
which it passes are counted, and this number represents 
the elevation or reduction of the temperature. 




Fig. io. — Brongniart s Pyrometer. 



A mercurial thermometer may have a scale extending from 
40 degrees below zero to 360 degrees above.* It is only 
between these two temperatures that mercury remains liquid. 
At a lower temperature it becomes solid ; at a higher it is 
gaseous, and in consequence it can no longer avail us in 
the circumstances. 

The alcoholic thermometer is employed for measuring 
excessively low temperatures, because this liquid does not 
freeze. As for very high temperatures, they are measured 
by pyrometers, a kind of thermometer dependent on the 
expansion of solid bodies. 

Thus in the manufacture of Sevres china M. Brongniart 
has used the thermometer represented in fig. 10 to estimate 
the high temperature of his furnaces. 

* In Fahrenheit's scale from 39° below zero to 680 above. 



MEASUREMENT OF HEAT. 39 

A plate of porcelain, placed in the oven, carries a bar of 
iron let ink) a groove, against the bottom of which one end 
of the iron bar rests, whilst the other end touches a bar of 
porcelain which passes through the wall of the furnace. 
This is pressed against the bar of iron by a lever. When it 
is heated the iron expands, and also the porcelain, but the 
lengthening of the latter may be neglected ; it is the iron 
which puts the lever in motion. The movement is trans- 
mitted by a ratchet and pinion to a needle* so constructed 
as to traverse the divisions of a dial. By this artifice the 
changes produced by heat in the length of the bar of iron 
are easily ascertained. 

The scale inscribed on the dial marks ioo° and zero 
(Centigrade) when the bar of iron is placed in boiling 
water or in melting ice; and the numbers rise to 1,500° 
because the iron remains solid until it reaches this high 
temperature. But such a thermometer cannot be compared 
with the mercurial thermometer, as different numbers are 
nearly always indicated by the two instruments when placed 
in the same circumstances. 

Physicists use, for precise experiments, air thermometers 
having some analogy with that of Galileo ; but it would be 
a digression to describe them more particularly. 

Having the thermometer, then, we are provided with a 
very delicate instrument, which will enable us to observe 
the phenomena of heat, much as the telescope enables the 
astronomer to observe celestial phenomena. In any experi- 
ment relative to heat we shall have to follow the displace- 
ment of the index, in order to note temperatures. 

5 . The measurement of Heat 

Our first application of the thermometer is for the purpose 
of fixing the unity of heat. 

As explained in our first chapter, we are able to measure 
the quantity of heat that a body disengages by the weight 
of ice that it is capable of melting. Evidently, to melt two 
kilogrammes of ice, twice the quantity of heat is wanted 
than is necessary to melt one. But such a unit is not 



40 THE PHENOMENA AND LAWS OF HEAT. 

chosen ; a more convenient one has been taken, to which 
is given the name of thermal unit. Take a kilogramme of 
water, and immerse the vessel containing it in melting ice ; 
a thermometer plunged into this water will, after some time, 
mark the zero of temperature. Take the vessel out of the 
ice and put it near the fire. As the water gradually gets 
warmer, we shall see the thermometer rise. When it has 
risen one degree, the water will have received a certain 
quantity of heat.' This quantity of heat is called the thermal 
unit. In melting one kilogramme of ice, 79 thermal units 
are consumed ; that is to say, 79 times the quantity of heat 
we have just defined. With all this heat, 79 kilogrammes 
of water could be raised from zero to one degree of 
temperature. 

The thermal units of heat must not be confounded with 
the thermometric degree. This last number is. simply a 
conventional notation • it is not a quantity, it is the indica 
tion of a state. Thoroughly to impress the difference on 
the reader's mind, we append the numbers of thermal units, 
which must be consumed to make one kilogramme of water 
undergo various modifications. 

Thermal units. 

To melt I kilogramme of ice 79 

To heat 1 kilogramme of water at zero to ioo° . . 101 
To reduce into vapour 1 kilogramme of water at 100 536 

Total quantity of heat consumed . . 716 

With this quantity of heat, 716 kilogrammes of water could 
have been raised from zero one degree. 

And let it not be imagined that this is all the heat con- 
tained in one kilogramme of water reduced into vapour at 
ioo° (Centigrade). Because if it is cooled it will disengage 
first 716 thermal units to reproduce ice at zero, and the ice 
could be cooled further. If one kilogramme of ice is cooled 
from zero to 100 degrees below, it disengages 50 thermal 
units, and it would disengage more if it were still further 
cooled. 

It is impossible to estimate the number of thermal units 



THE THERMAL UNIT. 4 1 

of heat contained by any body. We can only know the 
quantity of heat that must be given or taken away to bring 
it from some one state to some other, the thermometer 
guiding us to an appreciation of the change of state. 

We shall return to liquefaction and vaporization in a 
special chapter, and we shall further summarize what we 
have just now been obliged to explain. 

The definition of the thermal unit furnishes us with a 
conclusion which has an interesting relation to a result 
indicated in our first chapter. We have seen that a fire- 
machine is capable of raising a weight of one kilogramme 
to the height of 400 metres, causing a quantity of heat to 
disappear which would be capable of melting 12 grammes 
of ice. Now, for the fusion of one kilogramme of ice 79 
thermal units are required, as shown in the preceding table ; 
and for one gramme, being the thousandth part of a kilo- 
gramme, a thousand times less, multiplying which by 12, 
gives the quantity for 12 grammes. This, it will be found, 
is a little less than a thermal unit. If exactly one thermal 
unit were consumed, the kilogramme would be raised to the 
height of 425 metres. Hence this number 425 is called 
the mechanical equivalent of heat* 

* Tt is possible that in the progress of physics the figure 425 may be 
Uightly altered, but it is this figure which is at present generally used. 



CHAPTER III. 

SOURCES OF HEAT. 

I. Solar Heat — Terrestrial Heat. 

Any given body is called a source of heat when heat dis- 
engages from it, and the loss is at each instant repaired by 
a new production. 

The sun is the most beautiful and the most abundant of 
the sources of heat of which we are able to make use, M. 
Pouillet has invented an instrument with which he has 
measured the quantity of heat emitted by this body. It 
is a thermometer, the reservoir of which is inclosed in 
a very thin silver box, filled with water (fig. n). The 
tube of the thermometer projects from the box by one of 
its surfaces, and it is inclosed in a copper tube ? provided 
with a groove, so that the graduation may be visible. The 
other surface of the box is blackened with smoke ; this face 
should be exactly perpendicular to the direction of the 
tube. The instrument is exposed to the sun when there 
are no clouds to intercept its rays, and the box is turned in 
such a manner that the black face receives the solar rays 
perpendicularly. The rise in temperature is observed 
during five minutes, and a certain number of degrees 
thus obtained. The number of thermal units necessary to 
make the thennometer rise one degree being previously 
determined, a simple multiplication will give the additional 
number of such units gained by the instrument during its 
five minutes' exposure to the sun. To ascertain the real 
amount of heat that has reached the black surface, the 
preceding number must have added to it the heat that the 



HEAT OF THE SUN. 



43 



apparatus loses during five minutes by its own radiation 
towards the sky; for it is known that the celestial space 
exerts a cooling action on terrestrial objects. The exact 
amount of heat to be added is 
obtained by making in the dark 
another observation analogous to 
the preceding. 

But the whole amount of heat 
emanating from the sun is not yet 
ascertained, as some portion of 
it has been absorbed by the 
atmosphere. M. Pouillet has 
estimated this proportion by 
combining a great number of 
observations, and he has thus 
been able to calculate the quan- 
tity of heat that reaches the earth 
in one year. It is so great that 
ordinary figures would fail to 
convey an adequate idea of it. 
It must suffice to observe that 
it is capable of melting a mass 
of ice sufficient to envelope the 
globe to the depth of 30 metres 
(about 100 feet). Only one-half 
of this immense quantity of heat 
reaches the surface of the soil in 
consequence of absorption of the 
other moiety by the atmosphere. 

If we now wish to know the total quantity of heat emitted 
by the sun, not only towards our planet, but in all directions 
at once, an empty sphere must be conceived, the centre of 
which is the sun, while its surface is drawn through the 
centre of the earth. Astronomy teaches us that 2,300 millions 
of globes, as large as our earth, placed one against another, 
would be required to cover entirely this imaginary sphere. 
It follows that the whole quantity of heat emitted by the 
sun is 2,300 million times that which reaches our earth, 
and which we have just estimated. 




Fig. xx. — Pyrheliometer. 



44 THE PHENOMENA AND LAWS OF HEAT. 

How can such immensity be represented ? Let us make 
another effort. The sun is an enormous globe, 1,400 
thousand times larger than the earth. Imagine the surface 
of this body to be composed of a bed of ice 1,500 leagues 
in thickness. Yet this mass of ice would be melted by 
the heat which comes from the sun in one year. How 
should we fail to admire the power of the human mind, 
which, with the aid of so small an instrument as we have 
described, is able to resolve a question of such magnitude ; 
and, admiring this power, how should we fail in gratitude 
and adoration to its Divine Author ! 

The sun, then, is a permanent source of heat, and so are 
the stars ; but, being at an enormous distance, their heat 
produces no appreciable effect on our globe. There is still 
another permanent source of heat, viz., the globe itself. 

When a thermometer is lowered into the pit of a mine 
to various depths, it is found that the temperature rises 
about one degree Centigrade in thirty metres."* In accord- 
ance with this fact we find that water, when from certain 
deep sources, is always hot. At a depth of three kilometres 
(nearly two miles), if the preceding law remained exact, 
water would be in the state of vapour, always supposing it 
is not greatly compressed. When subjected to a certain 
amount of pressure, it remains fluid, even at temperatures 
above the boiling-point; but this is a phenomenon to which 
we shall recur hereafter. We are reduced to simple con- 
jectures as to the state of the earth at great depths, in con- 
sequence of the impossibility of making observations. It 
is probable that the centre of the globe is formed of an 
excessively hot fluid matter, similar to that, perhaps, which 
escapes from the crater of a volcano in the form of red-hot 
lava. 

2. Heat produced by chemical action. — Combustion. 

The sources of heat which we most frequently utilize are 
those called artificial, because we are free to make them act 
at will according to our wants. Ordinarily, they consist in 

* About 1° Fahrenheit in 50 feet English. 



COMBUSTION. 45 

the combustion of bodies. Combustion is the disengage- 
ment of heat and light, a phenomenon which accompanies 
the combination of certain substances. Generally, one of 
these substances is the oxygen of the air, and relative to 
this action it is called the supporter of combustion ; the 
other is the carbon of wood, or coal gas, which we call the 
combustible element. 

The number of combustible bodies is very great. We 
employ the preceding by preference, in consequence of their 
abundance and cheapness. We meet with the others in the 
practice of chemistry. 

When two different bodies combine to produce a new 
body, heat is always disengaged ; but for the production of 
light the combination must be effected with a peculiar 
energy. To illustrate this, moisten some quicklime with 
water \ the mass will heat, and vapour will be disengaged. 
A part of the water has combined with the lime ; in the act 
of combination heat has been produced, and it is this heat 
which causes the remainder of the water to become vapour. 
The paste which the mason prepares with lime for the pur- 
pose of mixing it with the sand to make mortar has there- 
fore the temperature of boiling water; it burns severely. 
Here the force which determines the combination of the 
lime with the water is not sufficiently energetic to cause the 
production of light. 

Now take a small piece of phosphorus ; place it in* a 
porcelain capsule, supported by an iron wire attached to a 
cork, and immerse the capsule in a vessel filled with the 
yellowish green gas known as chlorine. Combination will 
take place between the chlorine and the phosphorus ; the 
flask becoming filled with white fumes, which constitute the 
new body, and at the same time i flame will surround 
the phosphorus until it shall have completely disappeared. 
In this instance of combustion light is emitted by the 
union of the chlorine with the phosphorus, because the force 
which unites them is very energetic (fig. 12). 

We learn from this experiment the manner in which bodies 
may be arranged for combustion so as to collect their pro- 
duct. The same method must be employed if we wish 



4 5 



THE PHENOMENA AND LAWS OF HEAT. 



to examine the result of the combustion of carbon in 
oxygen. 

Instead of the phosphorus in the former experiment, put a 
piece of red-hot charcoal in a porcelain capsule, and immerse 
it in a bottle filled with oxygen. The charcoal now burns 

briskly; and when it has 
ceased burning, it will be 
easy to prove that the gas 
contained in the bottle is 
no longer oxygen, and that 
the piece of charcoal has 
diminished in weight. To 
place this beyond all doubt, 
introduce into the bottle 
a small lighted taper, and 
it will cease to burn, whilst 
if the bottle had contained 
oxygen it would burn with 
more activity than in the 
air. This is due to the 
fact that the charcoal has 
combined with the oxygen 
Fig. i2.— Combustion of Phosphorus in contained in the bottle, 
Chlorine. an( j produced a new sub- 

stance called carbonic acid 
gas. It is transparent and colourless, like air or oxygen, 
but it does not support combustion, a character which 
suffices to distinguish it. When carbon burns in our grates, 
the same chemical phenomena occur : the oxygen is pro- 
vided by the air, and the carbonic acid is carried up the 
chimney by the gaseous current. This current is formed ot 
this gas mixed with vapour of water, with nitrogen, which 
does not combine with carbon, and with smoke or finely 
divided carbon, of which the pulverulent particles are de- 
posited on the sides of the chimney, and there remain as 
soot. Smoke would not be formed if the oxygen was sup- 
plied to the fire in sufficient quantity, because all the carbon 
would burn : it indicates, therefore, an incomplete com- 
bustion. We remain contented with this, because we could 




COMBUSTION OF CARBON. 47 

not provide for complete combustion without adopting 
chimneys of a too costly construction. As to the ash from 
the fire, it is formed by the earthy substances which are 
mixed with the carbon, and which do not combine with 
oxygen. 

Carbon is charcoal in its state of greatest purity. Such is 
plumbago, commonly known under the name of black lead. 
Above all such is the diamond, most beautiful of all the 
precious stones, which is found in certain sands of the 
Indies and Brazil. The finest diamond in the French 
crown weighs scarcely twenty-eight grammes, and it is worth 
twelve millions of francs. It is known as the Regent 
Diamond, having been bought during the regency of the 
Due d'Orleans. If this magnificent jewel were to be placed 
in oxygen, after having been heated to redness, it would 
burn, away completely, consuming about 52 litres (nearly 11^ 
gallons) of oxygen, and producing about the same bulk of 
carbonic acid gas. 

The quantity of heat disengaged by the combustion of 
carbon is known, whether it be in the state of charcoal or in 
the state of diamond, because in both cases it is the same. 

One kilogramme of carbon, in whatever form, produces 
by burning in oxygen 8,000 thermal units, a quantity of heat 
capable of melting 100 kilogrammes of ice. This estima- 
tion will serve as a new illustration of the immense quantity 
of heat furnished by the sun. If the sun were covered with 
a bed of carbon burning in oxygen, and having a thickness 
of 27 kilometres (nearly 17 miles), the heat emitted by this 
gigantic conflagration would be equal to that really given 
out by the sun in one year. 

If we suppose the solar heat to be really due to such a 
conflagration, the sun being a globe of carbon, it would be 
entirely consumed in 5,000 years. As it has not changed 
in size or in magnificence since the creation of man, this 
hypothesis must be rejected. 

The convenient property of coal gas to burn in air has 
furnished us with a source of heat (and light), of which we 
have largely availed ourselves. The coal being heated in 
close retorts, a gas is produced, which passes by means of 



4 8 



THE PHENOMENA AND LAWS OF HEAT. 



pipes into an enormous reservoir, from which it is afterwards 
directed by subterranean channels to the various quarters of 
the town. Each gas-jet is adjusted to the end of a branch 
pipe from one of these channels, and a stop-cock closes it 
when it is not required to burn. When this tap is opened, 
a jet of gas escapes from the burner, consisting of carbon 
and hydrogen in combination. A little excess 
of pressure maintained in the gas reservoir, 
is sufficient to produce this jet. When a 
lighted match is brought near the jet, it is 
heated, the carbon separating from the hydro- 
gen ; the latter having a very great affinity, 
for atmospheric oxygen combines with it to 
form water in a state of vapour ; the carbon 
having a somewhat less affinity for oxygen 
combines next to produce carbonic acid gas. 
These two combinations produce so much 
heat, that a flame is kindled ; this flame con- 
tinues to heat the gas as it escapes from the 
burner. This gas burns in its turn, and thus 
the flame is maintained at the same intensity 
so long as the supply of gas continues. 

Let us examine the details of this com- 
bustion ; we shall receive some interesting 
and useful lessons. 

The flame has the form represented in 
figure 13, where the jet comes from one end 
of the straight tube. When the jet of gas 
escapes from a great number of small holes, 
as is often the case, the flame consists of 
an assemblage of many small flames, consti- 
tuted as we are about to describe. 

Hold a piece of paper over our simple flame at the base 
of the jet, and take it away before it shall have inflamed ; 
we shall see a black circular mark of the outline of the 
flame. From this we conclude that the centre of the flame 
is cool, and that there no combination takes place ; which 
is easily explained, when we say that the central parts of 
the jet are not in contact with air. The surrounding parts 



Fig. 13. 
Gas Flame. 



JET OF GAS. 49 

are, on the contrary, very hot; they form an envelope which 
has left its trace on our paper by carbonising it, and in 
which the combination with oxygen takes place, because 
diese parts are in contact with the air. This envelope is 
the luminous part of the flame, and we may notice that it 
consists of two parts, a fact which will lead us to discover 
the cause of its light. Present cautiously a very fine 
metallic wire, and you will observe that it becomes red-hot 
before reaching the luminous border of the flame. There- 
fore, there is a first stratum or layer, quite outside, in which 
the heat is very great ; it is in this stratum that the com* 
bustion becomes complete. Into the luminous stratum 
which it covers, the oxygen of the air does not penetrate 
sufficiently to effect the combustion of the whole of the 
carbon. The carbon then is set free for a short space ; it 
leaves the hydrogen, which, more mobile than itself, in- 
stantly combines with the oxygen of the air, and it is not 
until a little later, when arrived on the outside of the flame, 
that it burns in its turn, of course subject to the condition 
that the air is supplied in sufficient quantity on all sides ; if 
not, it will remain free, and, growing cool, will lose its 
affinity for oxygen, to maintain which, a high temperature is 
necessary ; in this case it will appear as smoke. It is in its 
passage from the inner to the outer part of the flame that 
the carbon, being free for a moment, and at the same time 
strongly heated by the outer stratum, becomes luminous. 

To sum up, there are three parts to be considered in a 
flame of gas : the dark central part, where there is no com- 
bustion, but where the carbon commences to separate from 
the hydrogen ; the luminous stratum, where the carbon is 
for a moment free and heated to whiteness ; lastly, the 
exterior bluish stratum, which is the hottest, and in which 
the combustion is completed. 

It will now be understood why the form of the burner is 
of such importance, and also the modification it should 
undergo, according as it is required to supply light or heat. 

If light be required, it is necessary to preserve the carbon 
for some instants from the action of the air, and yet not so 
long that it can become converted into smoke. If, on the 

D 



So 



THE PHENOMENA AND LAWS OF HEAT. 



contrary, heat be required, the carbon must be burned as 
quickly as possible, and not allowed to remain free. The 
celebrated German chemist, Bun sen, keeping these facts in 
view, has constructed a gas-burner which answers perfectly 
as a source of heat. 

The straight tube from which the gas-jet issues, is fixed in 
the axis of a wider tube, pierced near its base with a number 
of small holes (fig. 14). The air, entering by these holes, 
becomes intimately mixed with the coal gas, and, of course, 
the proportions of the mixture are regulated by the size of 





Fig. 14. — Bunsen's Gas Burner. 



Fig. 15. — Candle Flame. 



holes. It burns at the upper extremity of the wide tube, 
as shown in the engraving (fig, 14), with a very pale but 
nevertheless very hot flame. If the small air-holes be 
stopped up, the flame will become more luminous, and lose 
a proportionable amount of heat. This experiment demon- 



FLAME OF A CANDLE. 5 I 

strates, in the simplest manner, the exactitude of our 
reasoning. 

If the object of this little work was limited to the con- 
sideration of heat, what a variety of interesting phenomena 
would flame furnish us with. Here, for example, is an 
ordinary taper (fig. 15). Observe the little crater of white 
wax, in the centre of which rises the blackened wick, and 
at the same time inclines itself a little on one side. The 
heat of the flame is continually consuming, and as con- 
tinually renewing, the supply of melted wax, which is the 
combustible material. The parts farthest from the wick are 
the last to be melted ; the liquid collects at the base of the 
wick • it rises by the thousand small interstices of the 
twisted cotton which forms it, precisely as the coffee which 
you touch with a lump of sugar spreads immediately over 
the whole piece, and fills all its pores. This melted wax 
again is a combination of carbon and hydrogen ; heated in 
contact with air it is reduced to vapour, renewing inces- 
santly the supply of gas round the wick. After this point 
is reached all happens just as in the preceding experiment. 
You can distinguish in the flame of the taper the three 
parts which we have already studied in the gas flame. The 
wick remains blackened in the central part. Its bent 
extremity presents a little red-hot point in the exterior 
envelope ; at this point it is submitted to a very high 
temperature in contact with air, and is consumed entirely, 
which causes it to shorten as the taper is used. The curve 
of the wick is determined by the manner in which it is 
twisted. 

Among the numerous curious experiments which have 
been made with a burning candle, the following will serve to 
further confirm our theory. Frankland, being in the year 
1859 at Chamounix, at the foot of Mont Blanc, weighed 
six candles, and made them burn for one hour in the town; 
he weighed them again, and thus measured the loss in weight 
that they had sustained by the transformation of a certain 
quantity of the wax into carbonic acid gas and vapour ot 
water, during the combustion. Afterwards, he carried the 
same candles to the top of the mountain, about 12,000 feet 



52 THE PHENOMENA AND LAWS OF HEAT. 

in height, and again made them burn for an hour, whilst 
sheltered from the wind under a tent. The combustion 
seemed very feeble ; the flames were pale, and yet, on 
weighing the candles when returned, it was found that 
the quantity of wax consumed was nearly equal to that in 
the experiment at the foot of the mountain. Consequently, 
the combustion had in both cases an equal energy ; only at 
the top of Mont Blanc the pressure of the air is less by about 
one-half than it is in the valley. As a consequence, it is 
much more subtle ; it penetrates the flame more easily, and 
does not leave the carbon free for an instant. In accordance 
with this, it would be reasonable to suppose that, with a 
sufficient pressure, the flame of the same candles would 
become smoky, the compressed air being insufficiently mobile 
to allow it to enter the flame, and cooling the carbon 
sufficiently to prevent its burning. Dr Frankland, indeed, 
proved this to be the case somewhat later. 

In this manner an able observer not only explains natural 
phenomena which present themselves to him, but is also 
enabled to predicate other new phenomena, by a method of 
reasoning which is called induction. The verification of a 
foreseen result is a discovery which extends our knowledge 
beyond its own immediate bounds, and the joy of the 
philosopher when he has arrived at the desired end is so 
great that he soon forgets his fatigue. The celebrated 
physicist Rumford, referring to an experiment made in 
1798, acknowledged frankly that its success filled him with 
so much childish delight, that he felt it would be well to 
hide his joy if he was ambitious of the reputation of a 
grave philosopher.* But the contemplation of the marvels of 
nature suffices to render a man simple and modest ; his pride 
vanishes in presence of the unfolding grandeur of creation. 

Amongst the combustibles in use, hydrogen disengages 
most heat when combining with oxygen and producing 
water. With equal weights it disengages a quantity of heat 
more than four times that disengaged by carbon. It cannot, 
however, be advantageously employed, as it must be pre- 
pared by chemical processes and preserved in very large 
* Lessons on Heat, by Mr Tyndall. 



0XY-HYDR0GEN BLOWPIPE. 



53 



reservoirs, it being about thirteen times lighter than air bulk 
for bulk. ^ Pure oxygen must also be made for this purpose 
and kept in special reservoirs. It, therefore, is also an expen- 
sive agent to be employed in the production of heat, and for 
this reason it can only 
be used in certain cases, 
when the ordinary me- 
thods are insufficient. 
For example, M. Henri 
Sainte-Claire Deville has 
arranged the oxy-hydro- 
gen. blowpipe in the fol- 
lowing manner, for the 
fusion of platinum, a 
metal as precious as 
gold, sometimes used for 
minting, particularly in 
Russia, and which can- 
not be melted in an 
ordinary furnace. 

The oxygen is sup- 
plied by a copper pipe, 
terminated by a plati- 
num jet ; the hydrogen 
is supplied by a con- 
centric pipe, which in- 
closes the former and 
of which the extremity 
is also made of platinum 
(fig. 1 6). The gaseous 
mixture being kindled, 

burns with an excessively hot, pale-coloured flame. The 
jet is fixed in a narrow cylindrical hole pierced through 
a block of quicklime, which is placed like a lid on a 
vessel of lime. This vessel is pierced with holes at the 
base, so as to allow the vapour of water, which results from 
the combustion, to escape ; and it incloses a crucible, also 
made with quicklime, in which the metal is placed. The 
flame envelopes the crucible on all sides, and the platinum 




Fig. 16. — Oxy-hydrogen Blowpipe. 



54 THE PHENOMENA AND LAWS OF HEAT. 

melts. With five gallons of oxygen and ten gallons of 
hydrogen, one pound (Troy) of platinum may be melted. 

3. Heat produced by mechanical motion. 

Next to chemical action, mechanical movements offer us 
the most important means for the production of heat. Me- 
chanical motion, indeed, may be called the one universal 
source of heat throughout the creation, and from it all other 
means are derived. 

It is hardly possible to imagine a time when the fact that 
friction produces beat was unknown. Seneca observed that 
the shepherds procured fire by rubbing the extremity of one 
piece of hard wood in the cavity of another ; and this 
method is still in general use amongst savage tribes. Pivots 
in machinery become heated by friction against their 
cushions ; the nave of a wheel often takes fire by rapid 
turning. It is well known that fires have been caused on 
railways in this manner; to prevent such accidents some 
fatty matter has to be interposed between the rubbing sur- 
faces, and it requires to be frequently renewed. When the 
friction is very violent, there is a rending asunder of the 
parts, and the heatproduced becomes veiy great Thus, a steel 
wheel rubbing against a flint throws out particles of red-hot 
iron, and attempts have been made to use this method as a 
source of light for coal mines. When we strike a light with 
the old-fashioned flint and steel, we act in an analogous 
manner ; the particles of red-hot iron produced by friction 
and the shock of the stone fall upon the tinder and set it 
on fire. 

The turning of metals offers some remarkable examples 
of heat produced by friction. In one of Rumford's experi- 
ments, a casting intended for a cannon was fixed on the 
lathe to be bored. A cavity was made at one of its ends, 
and a blunt borer adjusted, so as to develop intense friction 
at the bottom of the cavity ; afterwards it was surrounded 
with water. The borer being put in action, hollowed out 
the piece, and the heat developed by this mechanical opera- 
tion reduced into vapour nearly nine quarts of water in two 




HEAT PRODUCED BY COMPRESSION. 55 

hours and a half. The hammer which strikes a piece of 
metal on the anvil heats it. A leaden bullet fired against an 
iron target has been known to melt in the act of striking. 
Solids, liquids, gases become heated when compressed; 
physicists have invented several experiments to exhibit this 
effect of compression. With solid or liquid bodies which 
are compressible to a small extent only — that is to say, 
which diminish very little in volume when under great pres- 
sure — the production of heat is small. Thus, to elevate the 
temperature of ordinary ether 6° only 
(about io° or 1 1° Fahr.), it is necessary to 
exert a pressure of 30 atmospheres — i.e., 
a pressure thirty times greater than that 
of the atmosphere. 

To appreciate this illustration, imagine 
a cylinder (fig. 17) containing a litre of 
liquid. This cylinder is a square deci- 
metre in size, and in consequence the 
height of the liquid is one decimetre. FlG - ^--Compressibility 

mi r 1 i t of liquids. 

The pressure of the atmosphere on the 
surface is equal to that of a piston weighing 103 kilo- 
grammes. This pressure is transmitted by the liquid to the 
sides of the cylinder, so that we may say the surface of the 
ether receives a pressure of 103 kilogrammes for each square 
decimetre. Now place a piston weighing 103 kilogrammes 
on the surface of the ether ; it will be compressed at the 
same time by the piston and the atmosphere ; the total 
pressure will be 206 kilogrammes, or 2 atmospheres. Fur- 
ther, put on the piston a weight of 1 03 kilogrammes, the 
total weight resting on the surface of the ether will be 309 
kilogrammes or 3 atmospheres. Suppose this multiplied by 
ten— i.e., to 3,090 kilogrammes — and you will have the idea 
of a pressure equal to 30 atmospheres. Well, under these 
conditions the temperature of the ether will be elevated by 
6° C, and the volume of the ether will be diminished by 
about 4 cubic centimetres. 

Gases, above all, become heated by compression, their 
volume also suffering considerable diminution. The ad- 
joining woodcut (fig. 18) represents an instrument depen- 



56 THE PHENOMENA AND LAWS OF HEAT. 

dent on this property. It consists of a glass cylinder, one 
extremity of which can be hermetically closed by a screw 
stopper, internally hollow, and in which also a 
piston works. To use it, the piston is placed at 
the open extremity of the cylinder, the stopper 
is unscrewed, and a piece of tinder put in its 
cavity. It is then screwed up again, and thus 
a certain quantity of air is inclosed in the appa- 
ratus in contact with the tinder. Now push the 
piston down sharply ; the bulk of this air will 
become very small, its elastic force very great, 
and it will spontaneously develop enough heat to 
make the tinder catch fire. On again opening 
the stopper, the tinder will be found burning. 

In all these experiments on heat produced by 
compression, it is necessary to exercise the 
mechanical effort with considerable quickness, 
without which the heat developed is commu- 
nicated to the neighbouring bodies, and its 
effect is no longer the same. By placing the 
body to be compressed in a reservoir, made ot 
some material which is a bad conductor of heat, 
such as glass, and by acting very quickly, the 
loss is but slight. 

4. Heat produced by mechanical work is 
equivalent to the work done. 

All the phenomena shown in the above expe- 
riments, the form of which may be infinitely 
varied, are marked by a common character, 
discovered a few years since, and which it is 
necessary should be clearly understood. But, 
first, it is indispensable to have some notion of 
Fig. 18. mechanics. 

Synng S e. ,on When a body is at rest, it cannot put itself 

in motion ; it is said to be inert ; it must be 

moved by some exterior cause, which is called force. So 

long as the force acts, the motion of the body is subject 



EQUIVALENT IN MECHANICAL WORK. 57 

to all the changes which depend on the method of action 
of the force ; but if it is suppressed, the body continues 
to move in a straight line uniformly, without power in itself 
to change this motion ; it shows inevitably, by virtue 
of the law of inertia, the impulse which the force has given 
it, and this lasts so long as a new force does not interfere. 
This is one of the fundamental properties of matter. 

It is impossible to find in the universe any body which is 
not in subjection to some force ; because all bodies attract 
each other, mutually following the law of universal gravi- 
tation, and it is this attraction which causes our globe to 
describe its annual circle round the sun, the moon to turn 
round our earth, and all the celestial bodies to move in 
space. 

In a great number of cases a body may sometimes appear 
to act as if no force were acting on it. For example, when 
we consider its state in relation to ourselves. Thus, a body 
suspended by a string is really, like ourselves, dragged along 
by the movement of the earth; but we say that it is at rest, 
because we abstract in idea this common motion, and thus 
simplify our reasoning. 

The body suspended by the cord is relatively at rest; and 
yet it is attracted by the earth, and if it does not rush to 
meet it, it is because the cord holds it. Where, in this ex- 
periment, do we find the principle of inertia ? The cord is 
stretched, because the earth attracts the body : the force 
which stretches the cord is called the weight of the body, 
which must not be confounded with gravity, the name by 
which we designate the cause of terrestrial attraction. The 
cord resists this tension, and this resistance may be said to 
neutralize the weight. The body is, therefore, submitted to 
the simultaneous action of two equal but contrary forces 
which destroy each other, and, in giving this explanation, a 
simple fact is expressed, such as observation reveals it. 

Now cut the cord ; its resistance no longer neutralizes the 
weight : we have thus suppressed one of the two forces ; the 
other remains, that which is due to gravity, and the body 
falls. Its movement is not uniform, so long as gravity alone 
actuates it. In the first second, it falls, let us say, through 



58 THE PHENOMENA AND LAWS OF HEAT. 

49 decimetres ; three times farther in the second, five times 
farther in the third, seven times farther in the fourth, and 
so on. When a body falls there is an expenditure of 
action, or work done, the exact measure of which is ascer- 
tained by multiplying the weight of the body by the distance 
through which it passes. If it weighs one kilogramme, and 
it falls from a height of 425 metres, it is said that a work is 
effected of 425 kilogram metres ; the body has then acquired 
a certain energy, of which one manifestation is the speed it 
possesses. 

If, by any artifice, the supposed body weighing one kilo- 
gramme were freed from the influence of gravity after its fall 
of 425 metres, the body would, by virtue of its inertia, con- 
tinue its vertical motion ; but in this case, the rate would be 
uniform, it would travel at about the rate of 91 metres per 
second. So long as this uniform motion continued, no fresh 
work would be spent ; the energy of the body would remain 
the same. The number 91 metres, measures its speed after 
the fall of 425 metres. Suppose that the assumed body 
weighing one kilogramme were a perfectly elastic ivory ball, 
and that, after having fallen at the distance of 4 25 metres, it 
strikes against a marble slab firmly fixed to the earth ; in this 
case the ball would rebound, and if the slab were perfectly 
horizontal, it would again rise to nearly the height of 425 
metres. Having reached its starting-point, it would remain 
an instant at rest ; and if at this instant gravitation ceased to 
act upon it, it would remain at rest for an indefinite period. 
Let us now inquire what really happened when the ball 
struck against the slab of marble. 

The ball has been flattened, and the slab has been 
slightly depressed at the point of contact. This fact may 
be demonstrated by rubbing a little grease on the surface ot 
the ball : after the shock, a circular spot will be seen on the 
slab, indicating that contact has taken place at a great 
number of points. Now, a spherical ball can only touch a 
plane in one point ; therefore the ball has been flattened, and 
its shape altered by the blow. On the other hand, the 
surface of the slab has been slightly depressed, but so very 
slightly, that we need not take it into account. This is a 



CONSERVATION OF FORCE. 59 

necessary condition to enable the ball to rebound to nearly 
the same height as that from which it fell. In other words, 
the marble slab being supposed perfectly inflexible, the flat- 
tening of the ball has alone to be considered. 

The ball has been progressively flattened while its speed 
was being annihilated, and at the same moment at which the 
flattening had become as great as possible, the motion ceased. 
The ball then resumed its primitive shape, still continuing 
in contact with the slab, from which it does not separate 
until it has entirely regained its form. At that moment it 
rebounds. 

As the ball rises, gravitation influences it so as to retard 
its motion by resistance. There is also the resistance of the 
air, which acts with the same effect as gravitation ; but this 
is very small, and we may neglect it. When the ball has 
come to a state of rest, at about the height of 425 metres, 
the force of the resistance overcome — in other words, the 
amount of work — has equalled 425 kilogrammetres. Thus 
the quantity of force developed by the fall of a body is 
capable of making the same body rise to a height equal to 
that of its fall ; and when this ascent is effected, it is entirely 
consumed. In general, we shall call spent work the effect 
of a force which makes a body move, and produced work the 
effect of a resistance which is surmounted by a body in 
movement. In our example, there is a work spent during 
the fall equal to 425 kilogrammetres, and equivalent work 
produced during the rise. 

The experiment we have just imagined is meant to explain 
the meaning of the conservation of force, which is one of the 
bases of mechanics. 

When a force has acted on a body and has put it in 
motion, it confers on it a certain energy which measures the 
work spent. The body loses this energy in the act of sur- 
mounting resistance, and produces work equal to the pre- 
ceding. It is this fact we have just observed. Very often a 
body loses its energy in a different way, — namely, by trans- 
mitting it to another body, and bringing itself to a state of 
rest. But then the bodies which have acquired this energy 
lose it in their turn by an equivalent production of work, 



60 THE PHENOMENA AND LAWS OF HEAT. 

when they again assume the mechanical state which they 
had at the moment of receiving the force. There are a 
great number of examples of this second mode of the con- 
servation of force ; but questions of this kind are treated in 
mechanics, and here we can only touch upon them in pass- 
ing, as a means of aiding us to comprehend the reasonings 
which follow. 

Again, take the body we have already assumed, weighing 
one kilogramme, and let it fall from the height of 425 metres 
into a body of water : this water will be briskly agitated, but 
will become gradually calm ; both weight and water are 
soon at rest. A motion has really been communicated to 
the water, but this motion vanishes without our being able 
to discover in the neighbouring bodies a production of work 
equivalent to 425 kilogrammetres. The quantity of action 
developed in the fall due to gravitation seems annihilated, 
without another force being visibly surmounted. The prin- 
ciple of the conservation of force does not seem to be 
maintained here. 

But let us examine this water as physicists. If we had 
placed a thermometer in it before the shock, we should have 
seen that there was an elevation of temperature. Thus 
water, in destroying the motion of bodies, heats spon- 
taneously ; therefore, the work spent has created heat. Is 
it not natural to suppose that this heat is just the equivalent 
of the quantity of work we cannot recover, and that the 
energy due to gravitation, instead of being annihilated, has 
been simply transformed, and has taken the form of another 
energy which w r e call heat? The motion of the mass has 
been replaced by a moiecular motion invisible to our eyes, 
but whose effects we may follow with the help of the ther- 
mometer. We therefore simply express a fact when we 
say that the heat created is equivalent to the mechanical 
work expended. 

5 . Mechanical equivalent of Heat, 

It results from the researches of a great number of savans, 
— amongst whom we may mention Mr Joule, in England, 
and M. Hirn, in France, — that the quantity of heat produced 



MECHANICAL EQUIVALENT OF HEAT. 6 1 

Jn the preceding experiment amounts to what it has been 
agreed to call one thermal unit. The appellation mechanical 
equivalent of heat has been given to the amount of 425 kilo- 
grammetres by M. Mayer, one of the founders of the new 
theory. If we perform the last experiment with a body of 
any weight whatsoever, falling into the water from any 
height, there will be a number of thermal units created equal 
to the number of times 425 contained in the number of kilo- 
grammetres expended : for example, if the body weighs two 
kilogrammes, and falls from a height of 850 metres, there 
will be 1 5700 kilogramme tres expended, and four thermal 
units created. 

We have already discussed the mechanical equivalent of 
heat (end of Chapter II.) in connexion with the subject of 
the production of work. Here, therefore, it will suffice to 
express the principle : when heat is made to overcome 
resistance, one thermal unit disappears, whilst 425 kilo- 
grammetres are produced ; and, on the other hand, when 
heat is created by mechanical motion, one thermal unit 
appears, whilst 425 kilogrammetres are expended. 

The same correlation exists between mechanical work and 
heat in all the cases in which percussion, fall, friction, or 
compression develop heat. In all experiments made with 
the object of measuring this correlation, there are two kinds 
of observations to make. The first are destined to make 
known, by calculations based on mechanical laws, the amount 
of work expended ; whilst the others serve to calculate, ac- 
cording to the laws of physics, the amount of heat created. 

To take a single example. Two or more plates of copper, 
fixed to a vertical axis, are completely submerged in water ; 
a string being wound on the axis, passes over a pulley, and 
is stretched by a weight (fig. 19); a thermometer is placed 
in the water, and the several weights of the water, the vessel, 
and the copper plates are known. Thus arranged,' the 
weight is allowed to descend. Gravitation is the motive 
force, and the work effected (travail moteur) can be 
measured by multiplying the weight by the distance it 
passes through in descending. The revolving plates agitate 
the water, and from the friction which takes place results 



62 



THE PHENOMENA AND LAWS OF HEAT. 



the disengagement of heat. The rise in temperature is 
marked by the thermometer, and thus we have all the data 
necessary to calculate the quantity of heat created. As to 
the work expended, there are several things to be con- 





sul 
Fig. 19. — Joule's Apparatus for producing heat by the friction of liquids. 

sidered. As the descending weight is stopped on reaching 
the earth, it might be thought sufficient to multiply the 
height of the fall by the value in kilogrammes of the weight. 
This product would in fact represent the available work, but 
it is not entirely expended by the friction of the revolving 
plates, and thus transformed into heat. A part is employed 
to overcome the resistance of the string, and that of the 
air, and another part to produce a little heat by the friction 
of the pulley, and that of the axis which carries the plates. 
These different portions of heat are not appreciated by the 
thermometer. Then, when the weight touches the earth, 
there is a blow or shock which creates heat, and which 
cannot be measured. It would be possible to estimate the 
quantity of work thus expended by certain contrivances 



MECHANICAL FORCE CONVERTED INTO HEAT. 63 

which it would be tedious to describe ; and by subtracting 
this sum from the whole available work produced by the 
descending weight, we should obtain the exact quantity 
to be compared with the heat measured by the thermo- 
meter. Such is the celebrated experiment made by Mr Joule 
about 1843. 

6. On some grand natural phe?iomen a in which mechanical 
force is converted into Meat 

Let us leave the laboratory, and take a few moments' rest 
from the fatigues of our long argument whilst contemplating 
the grand natural phenomena in which heat is engendered 
by causes similar to those we have just studied. 

For instance, we are travelling in a railway train. When- 
ever the train is about to stop we hear the grinding of the 
break, which is pressed against the wheels for the purpose 
of checking the motion. The train represents an animated 
mass travelling at a great speed : by the friction of the 
break, this mass is brought to rest, and heat is created on 
the rubbing surfaces. The work which has been spent in 
giving to the train its speed, is equivalent to this heat. The 
train starts again : the piston being pressed by the steam 
transmits the effort to the driving wheels of the engine ; these 
in their turn transmit it to the rails, and these react. It is this 
reaction of the rails which makes the train advance — a real 
exterior force pushing behind. If the friction ceased, and 
if the rail were perfectly horizontal, the train would behave 
like a body removed from the action of gravity, and which 
has received an impulsion. It would move uniformly by 
reason of its inertia, without a renewal of the impulsion or a 
fresh expenditure of motive work being necessary. But fric- 
tion always takes place, both at the axle-tree and on the 
rail. This is the reason, in the hypothesis of a horizontal 
load, that it is necessary to keep the steam in action to 
maintain the force. The motive work which the machine 
expends is said to be employed in overcoming friction. In 
reality, it is employed to create tlie heat which accompanies 
the friction ; and as this work is affected by the expenditure 



64 THE PHENOMENA AND LAWS OF HEAT. 

of the heat disengaged by the combustible in the furnace 
of the machine, it may be said that the function of the 
steam is to absorb the heat from the fire, and that of the 
rubbing surfaces to make an equal quantity of heat re- 
appear, which, of course, passes freely off. In this manner 
nothing is lost in nature, and the whole difficulty consists in 
following the transformation of forces. In the motion of a 
train over the horizontal road, the heat of the fire is con- 
tinually being transformed into mechanical work, and the 
work itself is being continually reconverted into frictional 
heat ; the motion of the atoms in combustion is transmitted 
to the larger masses of the train, and these masses in their 
turn transmit it to other atoms by the operation of friction. 

We can now understand the importance of greasing the 
axles of wheels sufficiently. If the person charged with 
this duty neglects to renew the grease contained in the 
boxes attached to the end of the axles, the friction becomes 
more intense, the amount of heat created augments, and 
the stoker is obliged to burn more coal or carbon. There 
is, at the same time, a risk of fire and increased consump- 
tion of the combustible materials, both which should be 
avoided. 

Let us visit the falls of the Rhine, the finest cataract in 
Europe. The waters of the stream fall from a height of 
20 metres (about 66 feet), with an expanse of 100 metres 
(about 330 feet). Nature has placed this immense obstacle 
across the bed of the river, and gravitation makes every 
kilogramme of water which arrives on its crest to fall the 
height of 20 metres. There is, therefore, a work of 20 kilo- 
grammetres converted into heat, exactly as in the case of the 
falling weight already discussed. The speed of our kilo- 
gramme of water increases in the fall ; afterwards this 
increase is extinguished in the whirl and turmoil of the 
waters, for the current of the river is not more rapid on one 
side than on the other at some distance from the fall. It is 
in the midst of the boiling waves at the base of the cataract 
that the mechanical effect of gravitation is replaced by heat. 
A mass of water of 21 kilogrammes engenders in this fall 
one thermal unit. Admit that 210 cubic metres of water fall 



HEAT PRODUCED BY FALLING WATER. 65 

in one second, an ordinary amount for a large river, we 
shall have 210,000 kilogrammes, and, in consequence, 
10,000 thermal units in one second ; in a day, 864,000,000 
thermal units will be thus created — that is to say, enough 
heat to melt 12,000 cubic metres of ice. 

This is not the only heat engendered by the movement 
of rivers. Throughout the whole course of the river, from 
its source down to the sea, its waters are descending a slight 
incline, drawn without cessation by gravitation, striking the 
banks and the inequalities of the bottom, dashing about 
without end ; a new cause of heat-production, a new example 
of the conservation of force. 

Let us visit the embouchure of the river, and stop for an 
instant on the shore of the ocean. Those immense waves 
that we see rise in the distance, with their frothy crests 
distinct against the horizon, must have been raised by some 
such powerful force as the wind. Gravitation makes them 
fall ; others rise and fall in their turn ; and all this agitation 
of the sea, all this turning and overturning, happens in 
accordance with a regular law. Those deep furrows like 
valleys advance parallel to the shore, reach it, and each 
liquid valley melts before our eyes in the form of a cataract, 
which exhausts itself in giving a final roar. Here is a spec- 
tacle of order in the midst of disorder. 

Have we hot in this infinitely extended agitation of the 
waters an incessant creation of heat, and are not sailors 
right in saying that the sea is warmer when rough than when 
it is calm ? Do not bathers, in receiving the shock of a 
wave, also receive a portion of the heat ? 

Among the movements of the ocean there is one of still 
greater magnitude than the preceding, and which has a very 
great influence on the calorific state of our globe. This is 
the tide. 

The mutual attraction between the earth and the moon 
would cause the two globes to fall together, were it not that 
an original impulsion opposed this force, and if the effect of 
this impulsion did not combine with that of the force to 
cause the moon to describe an almost circular orbit round 
the earth. This attractive force raises the ocean on the 



66 THE PHENOMENA AND LAWS OF HEAT. 

side next the moon, and also on the side diametrically 
opposite (fig. 20). As the earth turns on its axis in one 
day, the diameter of the earth in the direction of the moon 
describes on the surface of our globe a circular line from the 
west to the east, and the mass of water raised by the moon 
at the two extremities of this diameter would accurately 
follow this line if there were no continents. In consequence 



e 




Earth. Moon. 

Fig. 20.— Theory of Tides. 

of the irregularity of the shores and the action of the sun, 
the mountain of water only follows it in part ; but on the 
whole the effect remains the same. The tide rises twice a 
day, at different hours, because the movement of the moon 
round the earth follows a determined law. Now, what may 
be the mechanical effect of the tide ? The ocean is held 
back, so to speak, by the attraction of the moon, whilst the 
solid globe of the earth turns on itself. The ocean, there- 
fore, acts like a drag rubbing the surface of the solid globe, 
and from this friction heat is produced. Should we not 
suppose that this creation of heat is made at the cost of the 
earth's speed, if its motion is due to an original impulse, and 
if it has not within itself a motive force which constantly 
repairs its losses, and which produces mechanical work in 
proportion to the expenditure? This diminution in the 
speed of the diurnal rotation of our globe would cause an 
augmentation in the length of the day, but the increase is 
too minute, and man has been too short a time upon the 
earth, to enable us to estimate it. 

A learned Englishman, Professor William Thomson, has 
calculated the quantity of heat that would be engendered by 



THEORY OF ASTEROIDS. 6*J 

the friction of the earth if it were retarded by a drag until 
its motion was absolutely arrested, and he has found that it 
would be equal to the heat emitted by the sun in eighty- 
one days. 

We are tempted to pursue our excursion very far into the 
celestial world, but the gigantic phenomena of which we 
may get a glimpse are so intimately bound up with the sub- 
ject in hand, that we may well abandon ourselves for a 
moment to the pleasure of contemplating them. 

Shooting-stars, so called, have lately become very familiar 
objects. At certain periods of the year (August and 
November) they are very numerous. At Boston 240,000 
have been counted in nine hours. It is probable that they 
are little globes or masses which obey the law of gravitation 
like globes, but which, in consequence of their slight mass 
relatively to that of the earth, can be strongly attracted by 
her, when in her neighbourhood, enter her atmosphere, and 
there become heated to redness by the friction of the air. 
They then become a passing source of heat and light. Many 
are completely burned whilst traversing the atmosphere ; 
others explode like bombs ; a small number fall to the 
earth, and as they have an enormous speed, they penetrate 
the soil and bury themselves in it. These are called 
thunderbolts, and their fall is accompanied by circumstances 
which vividly impress those who witness it. Now, this 
shock produces heat, and this simple circumstance has led 
to a very singular hypothesis on the origin of solar heat. 

The universe, according to this hypothesis, is filled with 
asteroids, or little globes distributed in groups. Each group 
is supposed to gravitate round a centre, to which it gradually 
approaches. The sun is supposed to be in the centre of 
such a group, which constitutes the appearance called the 
zodiacal light. From this group an incessant shower of 
asteroids is rained upon the sun, and the falling projectiles 
create heat, according to the principles we have explained, 
at the instant of losing their motion. Professor Thomson 
has calculated that the quantity of matter which should fall 
every year upon the sun so as to maintain its temperature, 
would form on its surface a bed 66 feet in thickness. The 



68 THE PHENOMENA AND LAWS OF HEAT. 

bulk of the sun would therefore gradually increase, and it 
would result from this that, in fifty-three years, the length of 
time occupied in its rotation on its axis would be augmented 
by one hour, and that, in forty centuries, its apparent 
diameter would be augmented one-tenth of a second. It 
is probable that astronomers will sufficiently perfect their 
methods to be able to verify these indications of theory. 

The following tremendous hypothesis originated with Dr 
Mayer, and has been completed by other savans : — a It 
has been prophesied by the Apostle St Peter," said Pro- 
fessor Tyndall, in a lecture delivered in London in 1862, 
" that the elements shall be dissolved in fire. The simple 
motion of the earth comprehends all that is necessary and 
sufficient for the accomplishment of this prophecy." The 
earth revolves round the sun in one year at a speed of more 
than 1,100 miles a minute. If this motion were suddenly 
to cease, there would result from it a quantity of heat suf- 
ficient to reduce the whole terrestrial globe into vapour : if 
the earth fell into the sun like an asteroid, it would dis- 
engage as much heat by the shock as a globe of carbon 
6,000 times bigger would whilst burning in oxygen. 

Removing from our imaginary vision of the celestial 
regions, and re-entering our humble laboratory, shall we not 
be incited to reflect on the mechanism of atomic move- 
ments, and seek an explanation of chemical heat on the 
same principle? The very coal which we are daily burning 
in the midst of oxygen gas, does it not resemble a little 
sun, with its groups of asteroids continually showered upon 
it? May we not suppose that the atoms of oxygen, in the 
act of precipitating themselves on the carbon, to unite with 
it, disengage heat at the moment of losing their speed of 
motion ? We shall thus have assigned the same origin to 
the heat of common combustion and the solar heat. 

It must not be forgotten that all this is conjecture, or 
hypothesis; but this conception well illustrates the facts, 
and serves sufficiently well for their provisional connexion. 

Several other sources of heat remain to be glanced at, of 
which a more particular study can hardly be made in this 
book, because they belong to another branch of physics 



HEAT AND ELECTRICITY. 69 

and natural history : these are the electrical and the vital 
phenomena of heat. 

7. Heat developed by electricity. 

A thunderstorm is the grandest electrical phenomenon 
with which we are acquainted, and very often it produces 
the effects of intense heat. Masses of metal have had their 
surfaces melted ; the links of great chains have been fused 
together ; the hammer of a clock has been soldered by 
fusion to the bell ; bricks and stones have been vitrified ; 
the gilding on a piece of furniture has disappeared in vapour 
under the intense heat; and fine buildings and ships are 
often fired by lightning. 

Physical experimenters are able to imitate these effects 
on a small scale in their laboratories, and especially by using 
the Voltaic pile, which supplies us with a source of intense 
heat, and is susceptible of numerous interesting and useful 
applications. 

Reduced to the most simple state, a Voltaic pile consists 
of a plate of copper and a plate of zinc plunged into diluted 
acid, and kept from contact. If the exterior parts of the 
two plates are attached to the two ends of a single piece of 
copper wire, heat is disengaged both in the wire and in the 
water, whilst the zinc dissolves. What does the solution of 
the zinc consist in ? It is a chemical combination engen- 
dering heat ; it can be measured, and it has been found that 
one kilogramme of zinc, whilst dissolving in diluted sulphuric 
acid to form sulphate of zinc, disengages 560 thermal units. 
On measuring the amount of heat disengaged in the whole 
circuit formed by the pile and the copper wire which unites 
the two metallic plates, we find the same number of thermal 
units per kilogramme of zinc consumed. For example, if 
the pile exhibits one-halj^a third, a quarter of this number, 
the exterior wire will exhibit one-half, two-thirds, or three- 
quarters, so that the sum always gives the number 560. It 
is only the mode of distribution which changes, according 
to the length, thickness, and nature of the wire. From this 
law it is therefore evident that the heat disengaged in the 
circuit of the pile is of chemical origin ; that is to say, it is 



70 THE PHENOMENA AND LAWS OF HEAT. 

chemical action which engenders it : electricity is simply the 
force which presides over its distribution. 

If several elements analogous to the preceding are united 
by connecting the zinc of each element with the copper of 
the following one, a more energetic pile is obtained, the 
consumption of zinc being considerably augmented (fig. 21). 
By attaching to the last zinc a copper wire, terminating with 
a charcoal point, and to the last copper at the opposite 
extremity of the pile another wire similarly furnished, a 
light is obtained between the two points termed the Voltaic 
arc, the heat of which is the greatest known. With a pile 
of 600 elements, M. Despretz has in a few minutes melted 
about eight ounces troy of platinum. It is not unusual to 
see apparatus fitted up on these principles for illuminations 
on occasions of public rejoicing. 

8. Heat developed by animal and vegetable life. 

The vital phenomena occurring in organized beings, 
whether animals or vegetables, are a last source of heat. 
We have already stated in the first chapter that animals 
consume a part of their own heat, when they overcome 
resistances, when their muscles produce mechanical work. 
We have also seen how this expenditure of heat is repaired 
by food and respiration. The transformation undergone by 
the substances introduced into the body, are studied in 
natural history, under the name of phenomena of nutrition. 
Digestion supplies the blood with certain aliments which are 
essentially composed of carbon, hydrogen, and nitrogen, 
feebly bound together ; respiration introduces oxygen into 
this same blood, and then a kind of slow combustion goes 
on, creating heat, whilst the carbon and the hydrogen com- 
bine with the oxygen to form respectively carbonic acid and 
water. As for the nitrogen, it remains fixed in the tissues, 
contributing either to their growth or their maintenance. 
Thus animal heat results from chemical action. 

Vegetation lives in quite another manner. Under the 
influence of the sun's rays, the leaves of a tree or plant 
decompose water and carbonic acid ; the carbon and the 



VITAL HEAT. 



73 



hydrogen forming in the tissues of the plant those sub- 
stances sought by animals for food. We are led to believe 




Fig. 22.— Spathe of the Arum. 

that this decomposition spends heat, and that this heat 
comes from the sun. 



74 THE PHENOMENA AND LAWS OF HEAT. 

Thus, solar heat prepares our food in the plant. " We 
are," says Tyndall, " no longer in a poetical sense, but in a 
material sense, children of the sun." Definitively, animal 
heat is derived from solar heat. 

It would be a mistake, however, to conclude from this 
that animals alone, among living beings, produce heat, and 
that vegetables always consume it. The decomposition of 
carbonic acid and of water is not the only vital phenome- 
non occurring in a plant. There are chemical combinations 
continually taking place in the roots, buds, flowers, fruits, 
which result in the development of organs : these combi- 
nations are not very energetic; but they are nevertheless 
accompanied by a feeble disengagement of heat. The 
spathe of the common arum (fig. 22), at its flowering time, 
obtains a temperature 7 C. higher than that of the air; in 
Mauritius, the Arum cordifolium exhibits an increase of 30 C« 
An ordinary thermometer placed in the middle of the flower 
suffices to prove these effects, which are very sensible. But 
in most cases, it is necessary to employ the most delicate 
instruments analogous to the thermo-electric pile. 



CHAPTER IV. 

THE RADIATION OF HEAT. 

I. Reflection of Heat. — Burni?ig-mirrors. 

When two bodies of different temperatures are together in 
an otherwise empty space, the hotter of the two sends heat 
towards the other, and the propagation takes place in the 
straight lines that may be drawn from one body to the other. 
A particular movement which escapes our sight is trans- 
mitted from the hot body to the cold one ; the name of rays 
has been given to the lines of propagation, and radiation to 
the mode of distribution of heat. We have already noticed 
in the first chapter the analogy of this with the proceeding 
rays of light, and with the flow of liquid waves. The hypo- 
thesis of an ethereal medium, or elastic substance, through 
which the calorific waves are transmitted, and which fills the 
whole universe, has been imagined for the purpose of pic- 
turing the facts that observation has taught us concerning 
radiation. We propose to make a study of this subject, and 
supplement the insufficiency of our vision by reasoning, so 
that we may form an intelligible idea of the properties of a 
ray of heat. The hypothesis of the existence of an ethereal 
medium will simplify our explanation. 

WTien heat-rays fall on a large, polished, concave surface, 
there is found in front of this surface a point where the heat 
is most strongly concentrated. If the surface is presented 
to the sun, one may find, by means of a small piece of 
paper, a point where the light also is concentrated. It is 
exactly at the same point that the heat is strongest, and the 
latter may be sufficiently intense to burn the paper. A 



76 



THE PHENOMENA AND LAWS OF HEAT. 



polished metallic surface of this kind is called a burning- 
mirror, and the point at which the sun's heat is concen- 
trated is the focus. The phenomenon exhibited by the 
mirror, and from which results the formation of a focus of 
heat, is termed the reflection of heat. 




^rpY//W> 



Fig. 23. — Burning Mirror. 



A burning-mirror of large size is represented by fig. 23. 
Its frame carries three converging bars, which provide tke 
focus with a support to carry the substances to be submitted 
to the reflected rays. Very large burning-mirrors have been 
made, and it is on record that they have displayed remark- 
able power. The Tschirnhausen mirror, constructed in 



BURNING MIRRORS. 77 

1687, was made of copper ; it had a diameter of about 70 
inches, and its focus was about 6 feet from the surface. It 
melted copper and silver, and vitrified brick. In 1757, 
Bernieres constructed for Louis XV. a burning-mirror made 
of tinned glass, of much greater power. In this kind of 
mirror, a layer of tin-amalgam is deposited on the convex 
surface, and the rays which reach the concavity traverse the 
glass, and, being reflected back from the metallic surface, go 
forth again by traversing the glass a second time. There is 
certainly a small amount of reflection on the concave surface 
of the glass ; but its effect is much feebler than that from 
the tinned surface. Among celebrated experiments, we may 
also mention that of Mariotte, who ignited gunpowder with 
a mirror constructed of ice ; and, above all that, of Buffon, 
who succeeded in burning wood at a distance of 200 feet 
from the mirror. His apparatus was composed of a hundred 
flat mirrors of tinned glass, each a superficies of six inches 
square, and adjusted with hinges to a frame ; so that the 
whole constituted a spherical surface. This experiment 
shows the possibility of the well-known legend of Archi- 
medes. " That philosopher/' says the historian Zonaras, 
" having received the sun's rays on the surface of a mirror, 
with the help of these rays, gathered together and reflected 
by the imperviousness and polish of the mirror, inflamed the 
air, and lit a great flame, which he threw entirely over all the 
vessels that were anchored within the sphere of its activity."* 
This incident is supposed to have occurred at the siege of 
Syracuse by the Romans, whose vessels were burnt with the 
help of burning-mirrors. We have just seen the possibility 
of lighting wood at a great distance by joining together a 
number of mirrors ; but we can better understand the ex- 
pression of the historian, by supposing that Archimedes 
used mirrors to set on fire combustible substances, which 
were aftenvards thrown over the enemies' vessels. 

We shall now proceed to show that the laws of the 
reflection of heat are the same as those of the reflection ot 
light and sound. 

Take two concave mirrors of polished copper, exactly 
* Traite de Physique de Daguin. 



7 8 



THE PHENOMENA AND LAWS OF HEAT. 



alike. The reflecting surface is understood to be strictly 
spherical — that is to say, all its points are at an equal 
distance from a point called the centre of the curvature. 
An imaginary straight line passing through the centre and 
the middle of the surface is the axis of the mirror. To 
make the experiment we place our mirrors face to face (fig. 
24), in such a manner that their two axes exactly coincide. 




Now put some pieces of lighted charcoal in a grate of iron 
wire, and place this grate near one of the mirrors, in the 
line of the axis and at an equal distance from the mirror and 
from its centre of curvature. If we operate in the dark, we 
shall find, with the help of a small piece of paper, that a 
focus of light is produced before the second mirror at a 
point situated on its axis, at an equal distance from the 
mirror and from its centre. The explanation of this is as 



CORRELATION OF HEAT, LIGHT, AND SOUND. 79 

follows : — A ray starting from the incandescent carbon meets 
the first mirror ; it is reflected in a direction parallel to the 
axis, and necessarily strikes against the second mirror ; it is 
reflected again thence, and passes by the focus. All the 
rays of light received by the first mirror travel similarly, and 
meet in the focus of the second ; hence the concentration 
of light. If this reasoning is exact, we should be able to 
remove the mirrors to a greater distance apart, without 
changing the effect, and this indeed may be proved. 

At the focus of light opposite the second mirror place 
some tinder or gunpowder, taking care to interpose between 
the mirrors a wooden screen to intercept the rays. Directly 
the screen is removed, the inflammable matter will take fire, 
and we shall have proved, in a striking manner, that the 
focus of heat coincides with that of light. In a word, that 
heat-rays travel in the same direction as light-rays. 

Take away the charcoal, and put a watch in its place : we 
shall not hear the sound of its ticking in the neighbourhood 
of the second mirror at a spot taken by chance. But if we 
place our ear at the exact spot that our preceding experi- 
ments have determined to be the focus of the mirror, we 
shall hear it distinctly, although there may be a distance of 
several yards between the two mirrors. It is thus proved 
that the propagation of sound follows the same law as that 
of heat and light. What then is sound ? 

A sonorous body is animated by an oscillatory move- 
ment ; its entire mass is in vibration, and each of its 
vibrations produces on the surrounding air a pulsation 
which is transmitted to our ears. The impression received 
by this organ is followed by the sensation which we perspic- 
uously distinguish from other sensations by calling it sound. 
The direction in which a series of pulsations is transmitted 
through the air is called a sonorous ray. In any treatise 
devoted to the philosophy of sound will be found experi- 
mental proofs of this mode of transmission, which, more- 
over, is analogous to that of liquid waves. In our experi- 
ment, each sonorous ray reaches the ear after two reflections 
from our mirrors. 

The rays of heat and light behave like the rays of sound. 



80 THE PHENOMENA AND LAWS OF HEAT. 

Imagine, in place of the vibrations of the sonorous body, 
those of the ether contained in the source of such heat and 
light, and, in place of the pulsations propagated through 
the air, analogous pulsations propagated through the ether 
comprised between the two mirrors, and you will have a kind 
of picture of the radiation of heat and light 

2. Refraction of Heat. — Burning-glasses. 

The heat that falls on any body whatever is not entirely 
reflected. Generally, a portion is absorbed ; it is employed 
to warm the body, to raise its temperature, or even to melt 
it or to vaporize it. Another part traverses the body as 
light passes through the glass of our windows. The metals 
which we have used in the construction of mirrors com- 
pletely absorb that part of the heat which is not reflected ; 
they are not traversed by any ray, whether calorific or 
luminous. In other words, the metals are not transparent 
either to heat or light. For this reason they are employed 
as screens ; it is the same with wood and with stone. 

The study of the transmission of heat will conduct us to 
the fundamental laws of radiation, to those which explain the 
infinite variety of the phenomena of propagation. Already 
the study of reflection has taught us in what sense to accept 
the word ray, and how we can follow a ray in thought so as 
to explain the facts which we observe. It is sufficient to 
remark that we can feel the warmth of the suns rays 
through a pane of glass for it to be understood that glass is 
transparent to the sun's heat. Imagine a piece of clear 
glass having the form of a lens, and presenting two spherical 
convex faces, (fig. 25.) Place it in the sun, we shall find, 
with a piece of paper, a point where the light is concen- 
trated ) this point is on the side of the lens from which 
the rays of light and heat escape ; it is called the focus. 
This observation shows that a ray of light in traversing the 
lens deviates from its original direction ; we say that it is 
refracted, and we call this change in direction refraction. 
Now, the heat is equally concentrated at the same point ; 
because the paper takes fire there. If we place the bulb 



BURNING-GLASSES. 8 1 

of a thermometer in the focus, the mercury rises rapidly 
in the tube, a fact which shows the heating power to be 
very considerable ; whilst, if the thermometer is not exactly 
at the focus, the mercury rises only a few degrees. There- 
fore the heat rays are also refracted, and they follow the 
same route as those of light : a new result which, like that 
afforded by reflection, proves the analogy between heat 
and light. 

The property of burning-glasses was known long before 
the Christian era. It is indicated in the following dialogue 




Fig. 25*— Converging Leas. 

which occurs in the " Clouds " of Aristophanes: — "Have 
you seen at the druggists' shops the beautiful transparent 
stone which they use to produce fire ?" — " Do you mean 
glass?" — "I do." — "Come, what would you say, pray, it 
I were to take this when the clerk was entering the suit, and 
were to stand at a distance, in the direction of the sun, thus, 
and burn up all the writings?" 

The burning-glass of Tschirnhausen was about three feet 
in diameter, and it even melted gold. In 1763 experiments 
were made in England with a lens of ice, 10 feet in diameter, 
which was exposed to the sun, and in the focus of whieh 
gunpowder was ignited. Buffon constructed a liquid lens, 
by uniting by their edges two large plates of glass, having 
the shape of watch-glasses, so as to form a kind of lenticular 
trough, which was filled with liquid. The most powerful 

F 



82 THE PHENOMENA AND LAWS OF HEAT. 

effects were obtained by Bernieres and Trudaine in 1774, 
with a lens of this kind of about three feet six inches in 
diameter, and which was filled with alcohol. 

The figure 25 # represents an apparatus constructed 
according to a design published in the works of Lavoisier 
(t. iii.). With the help of an ingenious piece of mechan- 
ism, a single man could direct the lens, and follow the 
sun in such a manner as always to maintain the focus at 
the same point. Before meeting at the focus, the rays 
issuing from the large lens traversed a second smaller glass 
lens ; by this means they were brought into a narrower 
compass, and the focus became hotter. This burning- 
glass melted iron easily; platinum itself exhibited traces 
of fusion when exposed to its power. 

When very large glass lenses are required, such as those 
employed for lighthouse purposes, they are composed of a 
central lens, and of concentric crowns joined with isinglass. 
One of the surfaces is a plane; the other presents curvatures 
calculated for each crown in such a manner that all the 
solar rays are directed to exactly the same spot. These 
glasses are called lenses en echelons, in consequence of the 
projection made by each crown, as may be seen in the 
section (fig. 26), and their superiority depends upon the 
fact that, being less thick, they absorb less heat or light 
than lenses of the same diameter made of a single piece of 
glass. Buffon first suggested the idea of these lenses, but 
we owe their actual construction to Fresnel, one of the most 
celebrated physicists of our century. Their introduction 
into England is due to the late Sir David Brewster. 

The experiments made by means of mirrors and lenses, 
suitably interpreted, would enable us to reveal the law 
according to which every ray is reflected and refracted, 
paying attention only to the change in direction. But this 
investigation belongs naturally to a work on optics, or an 
explanation of luminous rays, which follow the same law. 
The study is, moreover, a purely geometrical one, as the 
direction of the transmitted rays is determined by the shape 
of the transparent body. We shall not follow this subject 
out here ; but in choosing for our transparent body a simpler 



HEAT AND LIGHT RAYS IDENTICAL. 



85 



form than that of the lens, we shall arrive at fresh results 
much more important in relation to the constitution of 
calorific rays. It is only by slow degrees that we arrive at 




m*ahdieio. 



HlLDIBRMD 



Fig. 26 — Lens. 



in advance 



truth. Each discovery we make is but a step 
which prepares for further progress. 

3, Physical identity of a ray of light with a ray of Heat — 

Luminous and calorific spectra. 

For a long time the analogy only of radiant light and 
heat was admitted : it is since the recent works of the 
Italian Melloni that we have been led to admit their 



86 THE PHENOMENA AND LAWS OF HEAT. 

identity. It is now generally believed that the sources oi 
heat and light send through the ether pulsations of only 
one kind, and that these pulsations may differ among them- 
selves according to the rapidity of their succession ; much 
as shrill sounds differ from deep sounds in the greater 
rapidity of the vibrations of the sonorous body. A ray 
which propagates in the ether a series of very rapid pulsa- 
tions acts specially on our eye, and determines there a 
sensation of light, because the nerve of the eye is organized 
to that end; on the contrary, a ray which propagates a 
series of slow pulsations, makes no impression on the eye, 
whilst it affects the nerves of our skin, and causes there a 
sensation of heat. But there is no essential difference 
between these two rays so far as the motion which they 
represent; mechanically, they are of the same kind; they 
differ only in their quality relatively to our senses. 

As the clear understanding of this conception is inti- 
mately connected with that of the propagation of sound, it 
may be useful to recall several experiments which have been 
made in the latter branch of study. First, let us suppose 
we are provided with a wheel made of copper, the circum- 
ference of which is cut into thirty-four equidistant teeth ; 
this wheel being capable of turning on its axis (fig. 27). It 
we press the card on its edge, we shall distinctly hear the 
noise each time the card touches a tooth. But turn the 
wheel faster and still faster, the blows will then follow 
upon each other closer and closer, and will soon produce a 
continuous sound. The number of turns made by the wheel 
in a second may be counted ; suppose it makes ten turns, 
there will then be 340 blows in a second, and as many 
pulsations communicated by the air to our ear. These 
pulsations succeed each other in the air at a distance of 
one metre, and at an interval of .^oth of a second ; so that 
when the 340th leaves the card, the first will have reached 
bodies 340 metres distant. The sound that we shall hear is 
nearly that called mi in the lowest octave of the violin (mi\). 

Further, if the wheel be made to rotate ten times faster, 
we shall have 3,400 pulsations per second. The distance 
between two consecutive pulsations in a sonorous ray will 



WAVES OF SOUND. 



87 



be one decimetre ; and when the 3,400th pulsation leaves 
the card, the first will have attained bodies placed at a 
distance of 340 metres, as before. The sound heard will be 
nearly la on the third octave above that marked by the 
preceding experiment (/tf 6 ). 

The distance which separates two consecutive pulsations, 
or sonorous waves, is called in physics the length of the 




Fig. 27. — Savart's Vv'ii _ci 



wave. The sharpest sounds, then, arise from the shortest 
waves ; in the preceding examples we had two waves, one 
measuring one metre, the other a decimetre. 

Now, fix on one and the same axis two wheels, having 
respectively 34 and 340 teeth (fig. 28,) and cause them to 
revolve ten times in a second. On touching the edges of 
these wheels alternately with the card, we shall produce, one 
after the other, the same sounds as before ; and a person 



88 THE PHENOMENA AND LAWS OF HEAT. 

gradually moving farther from the instrument will continue 
to hear them at the same interval of time ; they will appear 
to him to succeed each other in exactly the same manner. 
Of this fact we may assure ourselves any day, when we 
hear in the distance a musical air : the sounds reach our 
ear in the same order and with the same pitch, at whatever 
distance we may be ; the only difference is in the intensity : 
we do not, indeed, hear so well at a distance, but it is 
always the same air. The conclusion to be drawn from 
this is, that deep and sharp sounds travel with the same 
speed. With the apparatus we have just described, at a 
distance of 340 metres, two sonorous rays will reach our 
ears ; in one of them, that giving the bass sound, there will 
be 340 waves, measuring one metre ; in the other, that 
giving the sharp sound, there will be 3,400 waves, measur- 
ing one decimetre each. But in both cases alike a second 
of time will have elapsed between the departure of a wave 
and its reaching us. The distance travelled by any wave 
in a second is called the speed of the sound. It is 340 
metres in the air; it would be different in water, in the 
earth, or in a gas different from the air. 

This study of sound will enable us to distinguish between 
the different calorific or luminous rays. The source of the 
ray replaces the sonorous body ; the ether is supposed 
instead of the air. There are in ether waves of different 
lengths, which can also travel with the same speed side 
by side without being disconcerted. Only the numbers 
which measure heat and light are very different from those 
which measure sound. In a ray of red light, for example, 
it has been found that there are 16,000 waves per centi- 
metre; that its speed is 308,000 kilometres per second; 
and, as a consequence, that it makes, on entering the eye, 
more than 496 millions of millions of pulsations. The 
violet ray is estimated, in the same way, to make 699 
millions of millions of pulsations. 

These numbers are all the results of learned and pains- 
taking researches of distinguished experimenters. We have 
quoted them to express precisely our thought and to put 
the problem clearly before the reader ; but our real busi- 



THE PRISMATIC COLOURS. 



8 9 



ness at present is with the principal phenomena which have 
led to this theory. The following experiment shows that 
the rays emanating from the sun are separable into rays of 




Fig. 28.— Savart's Wheels. 



various qualities, in consequence of the inequality of the 
simultaneous waves which compose them. 

A hole is made in the shutter of a dark room, a mirror 
being placed outside to'reflect the solar rays into the room 
(fig- 2 9> frontispiece!) A piece of rock-salt, cut into the 
shape of a triangular prism, is placed in the direction of 
the rays in such a manner that they fall over only one angle 
of the prism. If this precaution be taken, no attention 
need be paid to the other angles ; that is to say, the piece 
of rock-salt may be of any shape we please, so that it is 
terminated by the two planes forming the angle. In tra- 
versing this angle the rays change their direction, deviating 
to the side of the room towards which the interior of the 
angle is turned. The refracted rays are received on a 
screen of white paper, producing the beautiful image seen 
in the coloured frontispiece. 



90 THE PHENOMENA AND LAWS OF HEAT. 

A luminous band composed of seven magnificent colours 
is displayed on the screen in the following order : — 

Violet^ indigo, blue, green, yellow, orange, red. 

The relative position of these colours shows that the 
deviation of the rays occurs in a decreasing ratio from violet 
to red. 

Newton, who first made this beautiful experiment, has 
called this image the solar spectrum. It is usually described 
in optics as being effected with a glass prism. We have 
chosen rock-salt for this experiment because our object 
is to study the calorific rays, which, as well as the rays of 
light, pass freely through this substance. If we leave for a 
certain time a very delicate thermometer, such as a small 
thermo-electric pile, to the separate action of each of the 
seven colours of the spectrum successively, starting from 
the violet, the temperature will be observed to rise, but not 
equally. As we approach the red, the increase of heat is 
greater in proportion. Now pass the red, and continue to 
move the thermometer in the same downward direction, but 
in the dark part. We shall now see that the heat becomes 
much greater than before, and that it reaches a maximum at 
a point situated a little beyond the red, and then decreases, 
though it is still appreciable to a considerable distance. It 
appears that there are in the solar rays some which are 
purely calorific, which are less refracted by the prism than 
the luminous rays, and which are distinguished amongst 
each other by the amount of their deviation and by their 
intensity. 

The luminous band, composed of seven colours, indicates 
to us that there are rays of light which are distinguished 
equally by their deviation and their manner of affecting our 
eyes. Those which are the most refracted produce the 
sensation of violet; then come six other rays, each pro- 
ducing a special sensation of colour. These luminous rays 
are also calorific. 

To complete our observations, it is necessary to make the 
rays of each colour fall successively on a second prism 
similar to the first. This we can do by means of a screen 



REFRANGIBILITY OF THE RAYS. 9 1 

provided with an aperture, as shown in fig. 30 {frontis- 
piece). We shall find in this case that the sepaiated rays 
undergo no further modification. The violet rays are simply 
deviated by the second prism ; they remain in the same 
state relatively to colour and heat, and it is the same with 
all the rest. The rays issuing from the first prism are called 
simple, and the rays coming from the sun are said to be 
composed of these simple rays : they separate in traversing 
the prism. The various simple rays are characterized by 
the deviation they undergo when falling in a similar manner 
on the same prism. Those which deviate the most are said 
to be most refrangible, or possessed of a greater refrangi- 
bility than the others. Instead of " deviated " we have, there- 
fore, sometimes used the word ' ; refracted." Thus the scale 
of refrangibility decreases from the violet to the red, and it 
decreases still more in the purely calorific rays. In fine, 
delicate experiments have shown, that if a simple luminous 
ray be submitted to a variety of operations which change 
its luminous intensity, it preserves a heating power which 
undergoes exactly the same changes. But there is not in 
consequence any reason to suppose that the luminous ray is 
continuously accompanied by a ray of heat distinct from 
itself. It is much simpler to admit that the same ray is 
capable of producing two effects, heat and light, and that 
there is no distinction save in relation to our senses. In 
this manner only can the observations be explained. 

We have still to discover how the theory of waves 
explains the separation of simple rays, the mixture of which 
constitutes solar rays. It will be sufficient to ascertain what 
happens on the face of the prism by which the rays enter, 
because the face from which they issue produces the same 
effect. The separation has commenced at the first face, and 
is simply continued at the second. 

Let us suppose two rays, the one violet, the other red, 
arriving together on the prism. The waves of the first are 
shorter than those of the second. In entering the prism 
they become still shorter, and they travel more slowly than 
in the air, because glass is a denser substance than air. It 
may be demonstrated mathematically that the diminution of 



92 THE PHENOMENA AND LAWS OF HEAT. 

these waves occasions the deviation of the rays, and causes 
them to continue their course more nearly perpendicular to 
the face on which they entered. Further, the red waves 
being longer than the violet, travel faster on the substance 
of the glass, and from this again results that the red ray is 
less refracted than the violet ray. What we have said con- 
cerning these two rays is applicable to the others. Hence 
they separate in their order of refrangibility. 

Our observation that the various waves of light do not 
travel with the same speed in one and the same transparent 
substance, may have appeared to the reader to be incon- 
sistent with what we have before ascertained concerning the 
waves of sound. But it must be remembered that it is only 
in ethereal space that the equality of speed is true ; in solid 
bodies the effect of the material molecules is to diminish 
the speed of the luminous or calorific rays in direct relation 
to the decrease in length. The analogy of sonorous with 
ethereal waves is complete. 

It is in consequence of the united efforts of physicists and 
geometricians that we are able to examine these questions ; 
and in pointing out their results we have simply wished to 
give an idea of the power which the human mind acquires 
when it is able to apply mathematical reasoning to the laws 
which have come under its observation in the study of 
natural phenomena. The mechanism of matter is dis- 
covered ; the molecules, with their harmonious movements, 
appear to the natural philosopher just as the worlds of 
celestial space appear to the astronomer who knows how to 
penetrate the mystery of its infinite depths. 



4. Sifting of the rays. 

In our study of transmission we used, by preference, a 
prism of rock-salt. If we had used one of glass, we should 
have obtained a solar spectrum identically the same. But 
we should not have found the less refrangible heat-rays, 
which, being invisible, we call dark. The conclusion to 
be drawn from this observation is, that the dark rays are 



SIFTING THE RAYS OF LIGHT. 93 

absorbed by the glass, the luminous rays alone being trans- 
mitted. Further, if we take a glass prism coloured red, for 
example, not only will the dark rays be arrested, but also a 
portion of the luminous rays. The spectrum is incomplete ; 
the red part is seen very well, but both blue and violet are 
missing. In general, most of the colours, with the exception 
of the red, are pale ; thus the red rays alone escape absorp- 
tion, the others being partially or completely absorbed. 
Indeed, some varieties of red glass may be found which let 
no other rays pass except those of this colour. The red in 
such a glass is called simple, whilst the red in the preceding 
is compound. 

It will now be easy to imagine the immense variety of 
effects produced by transmission through transparent bodies. 
We have simply to imagine, in place of the solar rays, a 
number of grains of all sizes, and, instead of transparent 
bodies, sieves having holes of all possible sizes \ we shall 
obtain a rough idea of the diversity in composition that the 
transmitted part may present. One sieve has holes larger 
than the grains, and so all pass freely through. In this way 
rock-salt allows rays of all kinds to traverse it. Another 
sieve has smaller holes, the larger grains are kept back. In 
the same way glass retains the dark rays. Another sieve 
has still smaller holes ; grains which passed before are now 
stopped : this illustrates the coloured glass, which retains 
the dark rays and all other luminous rays but the red. 

Under the last-named sieve place another similar one. It 
will give passage to all the grains which have traversed the 
first without retaining anything. To this we may compare 
the transmission of solar rays through two transparent plates 
of the same kind. The heat which falls on the second 
after issuing from the first passes freely, because it has been 
sifted, to make use of the original expression of Mr 
Tyndall. For example, solar rays fall on to a pane of glass ; 
the luminous rays are alone transmitted : they fall again on 
a second pane of glass, and are no more arrested ; so that 
the glass in the second case appears not to absorb heat, 
and in the first to absorb a great portion : this happens 
because the rays which reach the two glasses are not of the 



94 THE PHENOMENA AND LAWS OF HEAT. 

same kind. It follows that transmission depends both on 
the substance which forms the medium, and on the nature 
of the rays themselves. 

Again, let the two sieves, placed one above the other, be 
different; the first having large holes, the second having holes 
smaller than the grains. That which is passed through the 
first will be completely stopped by the second. Here is an 
illustration of solar rays, meeting first a red-glass medium, 
and then a blue one. The first glass has allowed the red 
rays only to pass, the second retains them ; so that in placing 
the two glasses one above the other, an opaque body will 
have been formed of two transparent bodies. 

Thus the transmission of rays is analogous to a process of 
sifting, but only when results are compared. The mechan- 
ism of the transmission is not in question, but it must not 
be supposed that a ray traverses a plate in the same way that 
a grain traverses a sieve. After having made the experiment 
with the solar spectrum, and having seen the prism separate 
the single rays which compose solar rays, it might naturally 
be asked, why our windows do not effect this separation ? 
why light entering white passes out white ? 

The answer to this question is simply as follows : — The 
surfaces of entrance and of issue in window-glass are 
parallel planes, whilst in the prism they form a certain 
angle. It is this which causes the difference in the two 
modes of transmission. In fact, if we unite two similar 
glass prisms so as to form a plate with parallel surfaces like 
a pane of glass (as fig. 31), we shall find that the solar rays 
will traverse it without being decomposed. This happens 
because the rays, after diverging from the first prism, are 
refracted in a contrary direction by the second. Thus they 
collect together again, and re-form on issuing rays similar to 
and parallel with those which arrived from the sun. 

Although separation does not take place, the absorption 
is the same as with the single prism, with this difference, 
that the absorbable rays are completely arrested by a suf- 
ficiently thick plate with parallel faces ; whilst in a prism, 
the absorption is never complete near the summit of the 
angle, in consequence of its small thickness. For this 



TRANSMISSION THROUGH THE PRISM. 



95 



reason, experiments in transmission and absorption are 
always made with plates. 

To recapitulate : Solar rays are composed of an infinity 
of simple rays, which are distinguished amongst each other 
by their refrangibility, if we wish simply to express the facts, 
or by the length of the wave, if we wish to employ the lan- 




Fig. 31. — Inverted Prisms. 

guage of the dynamic theory. When these rays strike 
against any given body, some part of them is reflected, 
another part may be transmitted ; the remainder are ab- 
sorbed, and these absorbed rays warm the body in which 
they are retained. 



5. Ltflnence of temperature on the emission of Heat. 

We learn to distinguish between the quantity of rays 
emitted by a given source of heat, and their quality. We 
shall first take into consideration their quantity. The more 
the temperature of the source is raised, the greater is the 
quantity of heat emitted ; always supposing that the body 
towards which radiation takes place, maintains a temperature 
constantly inferior to that of the source. If, on the con- 
trary, the temperature of this body, still remaining inferior 
to that of the source which does not vary, rises gradually, 



g6 THE PHENOMENA AND LAWS OF HEAT. 

the quantity of heat emitted by the source towards the body 
diminishes ; until the temperature of both having become 
equal, it ceases altogether. The action is reversed when 
the body is hotter than the source ; that is to say, the body 
now emits the heat ; it becomes a true source of heat 
itself. In this manner the sun sends us heat because its 
temperature is much above that of our globe ; but if the 
earth could acquire a temperature above that of the sun, it 
would in its turn emit heat towards the sun. The emission 
of heat by a body is therefore determined by the presence 
of a body having a lower temperature. For example : it is 
known that celestial space has a temperature of almost 2 oo° C. 
below zero ; the earth is therefore a source of heat relatively 
to space, towards which it incessantly radiates heat. A cloud 
comes in a given direction, the temperature of which is very 
far from being so low as that of space ; consequently, the 
earth will send a less number of rays in this direction than 
if the cloud did not exist. 

These principles will explain certain phenomena in which 
cold seems to be reflected like heat. These phenomena 
would lead a careless observer to imagine the existence of 
rays of cold distinct from rays of heat. No doubt a great 
error, as the following experiment will make evident. 

Take two metallic mirrors, arranged as fig. 24, and, 
instead of pieces of charcoal, place a few bits of ice in 
the grate at one of the foci. At the other focus expose the 
bulb of a very delicate thermometer. The mercury will 
now be observed to fall, whilst, if we cover the mirrors, 
there is no appreciable effect. The explanation is that the 
thermometer plays the part of a source of heat towards the 
ice. The rays which it sends towards the latter are, not only 
those which travel directly, and which are insensible in 
amount because of the distance, but those also which fall 
on the mirror next to the thermometer, and which, being 
reflected towards the opposite mirror, act by reflection from 
it on the ice. As there is nothing in the thermometer to 
repair the loss of its heat, its temperature decreases. It is 
true that bodies placed in the room around the apparatus, 
becoming then hotter than the thermometer, radiate towards 



EMISSION AND ABSORPTION. 



97 



it, and tend to repair this loss ; but it is only when its tem- 
perature is sufficiently low that the process of compensation 
can be established between the heat lost and the heat gained 
by the thermometer; it then remains stationary, its tempera- 
ture being a little higher than that of the ice, because a 
portion of its surface is submitted to the radiation from the 
room, which is supposed to be at a temperature above zero. 



6. Influence of the nature of the source on emission. — 
Correlation between emission and absorption. 

We have still to consider the quality of the rays emitted 
by a source of heat. First, there are dark sources, such as 
a piece of metal heated to 400 C, or a vessel of boiling 
water; such sources emit dark rays, 
which traverse rock-salt, but which 
are arrested by glass. Further, there 
are luminous sources which emit dark 
rays mixed with luminous rays ; the 
last are transmitted by glass, while 
the first are arrested. The composi- 
tion of the rays emitted, therefore, 
depends on the nature of the source 
of heat, and the properties of these 
rays change with it. Above all, the 
superficial covering of a body acting 
as a source of heat modifies their 
emission. Take a copper cube 
full of water, and make the water 
boil by means of a spirit lamp 
(fig. 32): the four vertical faces 
of the cube are at the tempera- 
ture of the boiling water ; but they 
differ in the state of their exterior 

surfaces. The first is blackened, the second is whitened, 
the third is of unpolished copper, and the fourth is polished 
copper. Apply to each of the faces of the cube in succes- 
sion the bulb of the thermometer, taking care to intercept 
the radiation of the lamp towards the thermometer by a 

G 




Fig. 32. — Leslie's Cube. 



9 s 



THE PHENOMENA AND LAWS OF HEAT. 



screen. Tt will be found that the thermometer will be 
unequally heated by the several faces, and if we follow the 
above order we shall find the temperature to diminish. The 
blackened side allows the greatest emission of the heat rays ; 
the polished one, on the contrary, renders the emission very 
feeble. The effect exerted by the state of the surface on 
the amount of emission may be proved without physical 
apparatus, by the comparison of a stove of cast-iron having 
a rough surface with a stove of polished 
iron. The first warms neighbouring bodies 
more than the second. Again, put similar 
weights of boiling water into two copper 
vessels, one of which is polished and the 
other blackened : you will find the first to 
cool more slowly than die second, and you 
will conclude from this that the emission 
from a black surface is greater than from a 
polished one. 

Inversely, if you put the same vessels full 
of cold water before the fire, the blackened 
one will get hot faster than the other. You 
will conclude from this that the absorption 
of heat by the first is greater than that by 
the second, so that you are led to think 
that the most absorbant bodies are also 
those which most easily emit heat. This 
correlation between emission and absorption 
is in fact established by numerous experi- 
ments, and it applies not only to the 
quantity of heat, but also to its quality. Thus a substance 
emits just such a group of rays, characterised by their refran- 
gibility, as it has the faculty of absorbing. We ought to find, 
according to our theory of the identity of heat and light, 
that the same law rules the absorption and the emission of 
light; and this, in fact, has been proved in experiments made 
of late years on the solar spectrum. The following is an 
example : Put a piece of sodium in the bright light pro- 
duced by the electricity of a Voltaic pile between two points 
of carbon, which light is called the Voltaic arc (fig. 33). 



:i 



Fig. 

Voltaic 



Arc. 



SPECTRUM OF THE SUN. 99 

The vapour of the sodium will become a source of light, 
and it will emit in abundance certain yellow rays of quite a 
peculiar intensity. If we cause these rays of light to traverse 
a prism, we obtain a spectrum which presents a characteristic 
yellow band. Instead of putting sodium in the Voltaic arc, 
reduce it into vapour at some distance, so that the rays of 
which you are about to form the spectrum shall traverse this 
vapour ; the spectrum will take the appearance represented 
in fig. 34 (frontispiece). The yellow rays are absorbed by 
the vapour, their place being occupied in the spectrum by a 
black band, indicating the absence of these rays. Such is 
the fundamental experiment which has led physicists to con- 
trive a method of observation with the help of which we 
may discover, according to the spectrum of a flame, either 
the nature of the vapours existing in it, or the nature of sub- 
stances traversed by the rays emitted by the flame before 
forming a spectrum. It is by this method that MM. Kirch- 
hoff and Bunsen have found that the atmosphere of the 
sun contains iron, magnesium, sodium, calcium, and some 
other metals. 



7. Influence of distance on radiant HeaL 

In our study of heat emitted from any source towards a 
given body, it is necessary to take the distance between 
them into account. If the body be gradually removed 
from the radiant source to double or treble the distance 
and so on successively, the heat received by it wili be four 
times, nine times, and so on progressively, smaller. The 
explanation is the same as that given in the case of sound, 
the intensity of which follows the same law. The body 
receives a certain number of pulsations on the part of its 
surface which is turned towards the radiant source. Imagine 
a hollow sphere, at the centre of which we have a source of 
heat, and the surface of which would pass through the body 
to which the heat radiates. We may, for simplicity, suppose 
that the latter is in the form of a disk, resting on the surface 
of the sphere, and that the entire sphere has a superficies a 
thousand times greater than that part of the disk turned 



IOO THE PHENOMENA AND LAWS OF HEAT. 

towards the source. In this case the sphere will receive a 
thousand times more rays than the body. 

Now suppose a sphere of twice the radius, with the body 
which we have assumed to be a disk placed upon it. The 
surface of this new sphere will be four thousand times 
greater than that of the body; but the same number of 
rays will arrive in the same time on the large sphere as on 
the smaller one. The large sphere will receive four thousand 
times more rays than the disk ; therefore the latter will 
receive four times less than before. 



8. Various applications of the preceding principles.— Dew.— 
Atmospheric water vapour. 

The laws of radiation explain a great number of facts 
open to common observation. For example : our bodies 
are submitted to the warming action of the sun by day, and 
during the night to the cooling action of the celestial space. 
Were the transition from a state of excessive heat to one of 
intense cold too sudden, it would be dangerous to health. 
To avert this danger, nature has given to the negro, for 
example, who lives under a tropical sun, so black a skin, 
that the abundant emission of his own heat has the effect 
of cooling him. On the other hand, as the absorption of 
the solar rays might be too great and neutralize the bene- 
ficent action of this emission, an oily perspiration lubricates 
his skin, so that the rays may be strongly reflected from its 
surface. In other places, man's intelligence enables him to 
find the resources necessary for his protection. The Arab, 
who requires protection from the sun when crossing the 
desert, and far from any shelter, envelopes himself in white 
wool. This stuff absorbs the solar rays less than any other; it 
reflects them in every direction, and diffuses them. Among 
ourselves, the use of white cotton preserves the heat of our 
bodies, because it is reflected interiorly by the parts of the 
cotton, which are only separated from the skin by a bed of 
air. Black clothing is as a rule hurtful, because in the sun 
it absorbs its heat strongly, and in the shade it emits on 
the contrary that of our bodies ; thus it rather aggravates 



LAWS OF RADIATION APPLIED. IOI 

the effects resulting from sudden changes of temperature 
than ameliorates them. White clothing is more suitable, on 
account of its comparatively feeble powers of absorption 
and emission. 

It is for the purpose of retarding the emission of their 
heat that nature has given to the animals living in polar 
regions a white coat ; and in other regions where the winter 
is rigorous, the coat is only white during the duration of that 
season. 

Opportunities for applying these very simple rules often 
occur. To preserve a body from the radiation of heat, give 
it a polished metallic surface if possible. Professor Tyndall 
mentions a very curious instance of this method of preset 
vation. A plank of wood, upon which had been written 
characters in gold, had been submitted to the radiation of a 
large fire ; the wood was carbonized all around the letters, 
but it was intact beneath them. The rays of heat had in 
fact been absorbed by the naked wood, and reflected by the 
gilding. 

On the contrary, to augment the absorption of radiant 
heat, give to the body a black surface. It is for this reason 
that gardeners paint the walls for espaliers black, so that the 
solar rays may be absorbed, and the heated walls may after- 
wards radiate towards the fruit ; the latter then receive both 
this heat and that which reaches them directly from the sun. 

We frequently utilize the property possessed by glass of 
absorbing the dark rays and allowing the luminous rays to 
pass. In foundries, the workmen observe the flow of the 
metal through screens of glass. The luminous rays alone 
reach their eyes, and these are the less ardent. Above all, 
the eyelids must be thus preserved ; the eye itself is less 
exposed, because its fluids stop the dark rays, and prevent 
the optic nerve from being burnt. In our gardens, the bell- 
glass we use to cover a young plant, acts by economizing 
the heat received from the sun. The luminous rays which 
have passed through the glass are absorbed by the earth and 
the plant ; these emit dark rays only, which cannot escape 
through the glass. The air confined over the plant may 
thus attain a temperature above that of the exterior air. 



102 THE PHENOMENA AND LAWS OF HEAT. 

The case is the same with our greenhouses, the solar rays 
having to traverse all the glass exposed to the sun. 

If we want to contemplate examples of greater magnitude, 
we have only to cast a coup d'ozil over our globe, considering 
it as a radiating, or, inversely, as an absorbing body. 

In the night, terrestrial bodies gradually lose the heat 
they have received from the sun during the day. If the 
sky is cloudy, the rays emitted from the surface of the earth 
are absorbed by the clouds, which are colder ; and this 
emission is smaller in the degree that the temperature of 
the clouds is higher. If the sky is clear, the emission of 
terrestrial heat is much more intense ; because the tempera- 
ture of the celestial space is excessively low ; but it is not 
the same in ' amount with all bodies. Water emits more 
heat than the earth, naked, or clad with verdure. Among 
various bodies exposed on the surface of the ground, some 
radiate more heat than others, according to their nature and 
situation in reference to the sun and air. Those, for instance, 
from which a part of the sky is hidden by walls, trees, 
elevations of the earth, or any other shelter, emit less than 
if they were not thus sheltered. With the same amount of 
exposure, the radiation of metals is inferior to that of stones, 
and these again to that of the green parts of vegetables. 
Evidently, the greater the amount of radiation of a body, the 
greater its decrease in temperature. We see then that the 
cooling of terrestrial bodies during the night is very unequal. 

The atmosphere is formed principally of air, a mixture of 
oxygen, of nitrogen, and of water in the state of transparent 
invisible gas ; the latter must not be confounded with fogs, 
which are composed of liquid water condensed into the 
form of small visible drops. The atmosphere absorbs but 
little dark heat, consequently its emission is very feeble ; 
hence it preserves, above a certain height, a temperature 
superior to that of the ground. Near the surface of the 
earth it becomes cool by reason of its contact with terres- 
trial substances, and its water vapour condenses on the 
surface of these bodies when their temperature is sufficiently 
low. The deposit of dew is unequal in different bodies. 
A piece of iron placed upon a stone in a meadow wil] 



DEW AND HOAR-FROST. 1 03 

remain dry whilst the stone will be slightly wet, and the 
grass will be covered with dew. The temperature in the 
daytime being less in the spring than in the summer, the 
nocturnal cooling of certain terrestrial substances, such as 
the green shoots of plants, may descend below the freezing 
point ; the water they contain solidifies, breaking the tissues, 
and the plants freeze. This phenomenon is called a white 
frost. In these cases the dew is no longer deposited in the 
form of liquid drops, but in that of needles of ice, which form 
the hoar-frost. 

In Bengal, where the diurnal temperature is very elevated, 
ice may nevertheless be obtained by nocturnal radiation, 
some such artifice as the following being adopted as means : 
Rather shallow pits are filled with straw, which is a bad 
conductor of heat, and on this straw are placed flat open 
vessels containing water freed from air by being boiled. The 
water emits its heat freely, the straw preserves it from the 
heat of the soil, and ice is formed. It has been noticed 
that the most favourable nights for the production of ice 
are those during which there is but little dew, the sky being 
moreover clear and without clouds. Evidently, if there is 
but little dew, the atmosphere is only slightly humid. Thus 
the absence of vapour of water in the air is favourable to 
nocturnal radiation. It has been known, indeed, since the 
researches of Professor Tyndall, that water vapour absorbs 
7 2 times more heat than the dry air with which it is mixed. 
The rays emitted by the earth traverse, therefore, dry air 
more freely than humid air, and nocturnal cooling augments 
with the dryness of the air. 

The white frosts of the spring are fatal to agriculturists ; 
and, ignorant of the true cause of this phenomenon, they 
have often accused the stars of exercising an influence on 
the earth which it is impossible they should have. As white 
frosts happen when the air is very clear and very dry, the 
moon shines in the sky with great brilliancy, and naturally 
attracts our attention ; but there is no more reason to attri- 
bute a cooling action to her than a putrefying one, which 
has been also done, because it has been noticed that animal 
substances putrefy more rapidly under the same circum- 



104 THE PHENOMENA AND LAWS OF HEAT. 

stances : it has not been noticed that these substances 
become abundantly covered with dew, and that it is the 
water which causes the putrefaction. Far from cooling the 
earth, the moon warms it by reflecting the rays of heat from 
the sun, as Melloni discovered with the help of very delicate 
instruments. 

Now that white frost is explained by radiation, it is easy 
to understand the reason for having recourse to such means 
of protection as the thin matting with which gardeners 
cover delicate plants ; also, the practice of protecting vines 
with clouds of smoke caused by lighting fires with resinous 
substances at that advanced hour of the night when the 
temperature approaches zero. This practice, which ought 
to be generally adopted in France, has been long known in 
several other countries, particularly in Peru. 

The bodies which cool most during the night are also 
heated most in the day, as soon as the solar rays reach 
them. Vegetables appear to be an exception, but it must be 
remembered that the water they contain evaporates, and 
that evaporation consumes heat, which cannot then serve 
to raise their temperature. Dry earth exposed to the sun 
may acquire from 20 to 40 C. of heat more than the air, 
and black substances take an excess of temperature still 
greater. These facts illustrate the correlation which exists 
between the emission and absorption of heat. In winter 
time the solar rays determine the partial fusion of the snow 
which covers the earth; this fusion prevents the tempera- 
ture of the soil from rising, and gives to it a humidity 
highly favourable to vegetation. The fusion is very slow, 
in consequence of the feeble absorption of the rays. To 
convince one's self of this it is sufficient to throw a little 
charcoal dust on the snow ; the fusion becomes rapid around 
each grain of charcoal, because it absorbs much heat, and 
the track of the dust is soon drawn on the snow by a deep 
furrow. The snow absorbs dark rays better than luminous 
rays, and for this reason it melts faster under the trees ; for 
when trees have been heated by the solar rays, they radiate 
in their turn the dark heat towards the snow which lies 
beneath their shadow. 



ATMOSPHERIC DRYNESS. IO5 

The preservative character of snow appears again at 
flight, when the sky is clear. The white carpet which covers 
our fields radiates feebly towards the celestial spaces ; its 
temperature falls to a little below zero (3 2° Fahrenheit), and 
it prevents the plants buried in the ground from being too 
severely cooled. 

We shall often again be called upon to admire the part 
played by water on the surface of the earth. Here we can 
only consider its relations to radiant heat, and we shall 
conclude this chapter with some observations on atmos- 
pheric water-vapour, feeling certain that it will throw a 
new light on the physics of the globe. 

According to the experiments of Professor Tyndall, the 
water-vapour that a certain volume of air may contain 
absorbs about 72 times more heat than that air absorbs 
when dry. This applies equally to the quantities of heat 
emitted under similar circumstances. 

At the bottom of a valley, with a river course, the air is 
necessarily more humid than on an elevated plateau or 
mountain. In the night, after a fine summer's day, when 
the sun disappears behind the hills, the valley is plunged in 
obscurity and deprived of solar rays before the surrounding 
heights. The humid air which it contains immediately 
begins radiating towards the celestial space, and the water- 
vapour, above all, renders this action very intense. The 
rapid radiation, in its turn, is the cause of a sudden cooling, 
owing to which the vapours condense in small imperceptible 
drops, which fall like a fine rain, though the sky is free of 
cloud. This is the evening damp. 

As we rise in mountainous countries, we meet with strata 
of drier air, and they absorb less of the solar rays. The 
traveller who crosses during sunshine the glaciers of the 
Alps experiences the effects of the feeble absorption. With 
his feet on the ice, he feels throughout his whole body an in- 
supportable heat. The solar rays freely traversing the dry 
air are absorbed by his garments, which become oppressively 
hot. The air is cold, nevertheless, because there is not 
enough vapour in it to retain the heat; and when a traveller 
passes from sunshine into shade, its excessive cold is felt 



Xo6 THE PHENOMENA AND LAWS OF HEAT. 

It may now be easily understood why burning mirrors and 
glasses are so much more powerful on the mountains or, 
more generally, in a dry atmosphere. 

The dryness of the air is, therefore, favourable to the 
heating of terrestrial substances during the day, and to their 
cooling during the night. Also, the driest countries suffer 
the greatest variations in climate. In the Sahara, the sand 
is burning, the atmosphere is like fire during the day, and 
the cold at night is excessive ; ice even is formed. We find 
in the account of a journey recently made in Asia and 
in Oceania, by the Count Henry Russel-Killough, many 
curious details about the excessive temperature of Siberia, 
of Thibet, and of Australia, countries where the aridity is 
excessive at certain seasons of the year. 

One day at Ka'insk, in Siberia, the temperature fell, in 
four hours, from zero to fifteen degrees below it;* some 
days afterwards the mercury of the thermometer sank com- 
pletely into the bulb, which seemed half empty, yet the tube 
itself marked thirty-five degrees below zero, f Tt will not 
surprise any to hear that, on the same day. brandy was 
frozen beneath hay and fur. il All this," says the author, 
" happened under the most dazzling sun. Clouds are im- 
possible at a temperature when vapour is turned into stone, 
and the blood freezes in one's veins.'' 

In Australia, there is sometimes a difference of 90 F. 
between the highest and lowest points of temperature 
marked by the thermometer. Our traveller has observed 
120 F. in the shade, and 147 in the sun. "The mor- 
tality," says he, " especially of children, became alarming ; 
birds fell from the trees as if thunderstruck, whilst others 
allowed themselves to be taken by the hand or came to 
quench their thirst in the interiors of the houses. Plants 
were burnt to the point of falling when touched, like the 
ash of a cigar." Captain Sturt has observed in Central 
Australia 129 F. in the shade and 160 in the sun. 

* From 32 to 5 Fahrenheit. f Equal to 31 Fahrenheit. 



CHAPTER V. 

CONDUCTION OF HEAT. 

I. Bodies which are good conductors. 

When we submit a part only of a body to the action of a 
source of heat, this part heats progressively the remainder oi 
the body. This propagation of heat from one part to 
another in the interior of a body is called conduction. It 
proceeds very slowly, in which respect it differs much from 
radiation. . 

Conduction is easily explained by regarding each mole- 
cule of the interior of the body as submitted to the heating 
action of molecules hotter than itself, and to the cooling 
action of colder molecules. The molecule is in equilibrium 
when these two contrary actions, which follow the laws of 
radiation, exactly counterbalance each other. The mole- 
cules placed on the surface of the body are, moreover, sub- 
mitted to the action of exterior substances ; and here we 
must distinguish between two different actions, the radiation 
between the surface and distant objects, and the passage of 
heat from the surface to substances which touch it, or reci- 
procally, according as the substance is hotter or colder than 
these objects. This passage is a phenomenon of conduc- 
tion, because it is effected from molecule to molecule ; it 
differs from interior conduction, inasmuch as the molecules 
put into play are of different natures ; it is distinguished by 
the name " exterior conduction." When a body is warmed 
by a source of heat in one of its parts, radiation and conduc- 
tion determine a cooling action, and each part cf the body 



I08 THE PHENOMENA AND LAWS OF HKAT. 

acquires a stationary temperature when this action exactly 
compensates the heating action of the source. 

We proceed to describe a very simple experiment, show- 
ing the conductibility of solid bodies. Two bars of the 
same size, but different material, are placed as in fig. 35, 
the wooden balls being fastened to them by means of wax. 
At the point where the two bars join a spirit lamp is kindled, 
and the wooden balls will be seen to fall one after the other, 
starting from the point heated. Now, when a ball falls, it 




Fig. 35. — Conductibility of sol id bodies. 

is because the wax which held it is melted, and this again is 
because the fusing point of the wax has been attained by 
the adjoining part of the bar. If one of the bars is iron 
and the other copper, it will be seen that a greater number 
of balls fall from the copper than from the iron ; therefore, 
heat propagates farther in the copper. Hence, copper is 
called a better conductor than iron. 

The conducting powers of a number of substances may 
be compared by adjusting equal bars of these substances to 
the vertical side of a metallic trough (fig. 36.) All the bars 
being covered with wax, the trough is filled with boiling 
water, and the wax melts to a greater or less distance from 
the trough, according to the conductibility of each indi- 
vidual bar. By such observations it has been easy to 
arrange a variety of bodies in a decreasing scale of conduc- 
tibility as follows : — 

SILVER. BRASS. LEAD. 

COPPER. TIN. PLATINUM. 

GOLD. IRON. BISMUTH. 



EXPERIMENTS ON CONDUCTION. IO9 

A very trifling observation of daily occurrence is sufficient 
to remind us of the comparatively great conducting power 
of silver. Plunge in the same vessel of boiling water a 
silver spoon and a pewter or iron one. It will be found 
that the handle of the first will get hotter than that of the 
other. 




Fig. 36 — Ingenhous's Apparatus. 

In experiments on conduction, care must be taken not to 
confound the intensity of the heating with its rapidity. 
Conduction alone determines the first, whilst the second is 
a complex effect of conduction and another property pos- 
sessed by bodies of which we shall treat in another section. 
Thus bismuth is a worse conductor than iron, and yet, 
with the above apparatus (fig. 36), we should see the wax 
melt quicker on the bismuth than on the iron. It neverthe- 
less conducts less, because the wax which covers it melts to 
a less distance ; thus the quantity of heat conducted is less. 
The same remark applies to our first experiment. It is the 
distance from the heated extremity at which the last ball 
that drops was fixed that must be noted, and not the 
rapidity of the successive falls. 

Here is another experiment which at first sight may 
appear paradoxical, and which reproduces the same phe- 
nomena under another form. Place on the lid of a vessel 
full of boiling water two small solid cylinders, one of which 
is of bismuth, and the other of iron. It is understood that 
these cylinders are each of the same dimensions as the 
other, and that their upper extremities are covered with 



no 



THE PHENOMENA AND LAWS OF HEAT 



wax (fig. 37). The heat from the vessel will gradually 
extend through the cylinders, and will at last melt the wax. 
The fusion commences on the bismuth because it conducts 
more quickly, not because it conducts more heat. We shall 
see by and by that, to raise equal weights of these two 
substances through the same number of degrees, we require 
four times more heat for the iron than for the bismuth. In 
order, therefore, that the temperature at which the wax melts 




Fig. 37. — Conductibility of Iron and Bismuth. 



may be attained at the upper extremities of the cylinders, it 
is obviously necessary that the iron should have transmitted 
more heat than the bismuth. Suppose the quantity of metal 
in each case is one gramme ; then we say the gramme of 
iron will have received four times more heat than the 
gramme of bismuth before fusion commences, and for this 
very reason it has taken longer to get hot. 

Substances which are good conductors of heat, such as 
the various metals, feel cold to the touch. It is because the 
hand that touches them, having a higher temperature, owing 
to the natural heat of the blood, gives up heat to them by 
virtue of conduction \ and this heat being rapidly diffused 
throughout their mass, the loss of heat which the hand 



EXPERIMENTS ON CONDUCTION. I j i 

suffers is continually repeated, and becomes very sensible. 
But, on the other hand, those metals are not always the 
best conductors which appear coldest to the touch. The 
effect depends on the rapidity of the transmission, as well 
as on the conductibility : and the one phenomenon is as 
complex as the other. Nevertheless, the great differences 
in conductibility may be recognised by the simple contact 
of the hand. Thus, wood appears less cold than marble, 
and marble less so than metal. The order in which we 
have named these three substances is that of their relatively 
increasing conductibility. It is supposed that the three 
bodies are touched after attaining the same 
temperature by remaining for some time to- 
gether in the same room. 

The same reasoning applies also to the fol- 
lowing experiment. Round a cylinder con- 
structed one half of copper, the other half of 
wood, roll a sheet of paper (fig. 38), and plunge 
the white surface into a flame for some moments. 
The fact of the cylinder being composed of 
two different substances will soon make itself FroT^s. 
apparent. That part of the paper which covers Double cylinder 
the wood is seen carbonized ; that which covers ° C woocL an 
the copper remains white. This takes place 
because the copper, a good conductor, takes the heat from 
the paper as fast as it comes from the flame, whilst the 
wood allows it to accumulate. 

We also find a very curious example of conductibility in 
a property of metallic gauze, which has given rise to one 
of the most beneficent applications of scientific knowledge 
in practical life. If we place a piece of wire gauze over 
the flame of a jet of gas (fig. 39), the flame does not 
traverse the gauze ; the exterior stratum, or hottest part of 
the flame (as shown in Chapter III.), reddens the gauze, 
and the circle of fire thus traced is a new demonstration of 
the constitution of flame. No combustion takes place above 
the gauze, which, however, gives free passage to the com- 
bustible gas. This may be proved by presenting a lighted 
match above the gauze, when we shall see the gas burn ; 




112 'J'HE PHENOMENA AND LAWS OF HEAT. 

or we may extinguish the gas at the jet, and replacing the 
gauze, we can light it above without the flame passing 
below. The metallic gauze, therefore, intercepts the heat 





Fig. 39. — Properties of Metallic Gauze. 

only ; it sufficiently cools the ignited gas to prevent the 
combustion commenced at one side from taking place at 
the other. This cooling is due to the conductibility of the 
copper wires which compose the gauze, exactly as in the 
preceding experiment. 

By taking advantage of this property of metallic gauze 
Sir Humphry Davy constructed the safety-lamp for the pur- 
pose of protecting coal-miners from the terrible accidents 
to which they are exposed by the disengagement of fire- 
damp. Each blow of the pickaxe which disengages a block 
of coal liberates a certain quantity of this substance, a gas 
formed of carbon and hydrogen, which exists naturally in 
the interstices of the coal. As the mine is always at a 
considerable depth in the earth, it is worked by digging 
galleries, and consequently the fire-damp (as miners call 
this gas) remains in them, mixed with a proportion of 
atmospheric air. If a light approaches this mixture, explo- 
sion takes place, in consequence of the rapid combination 
of the carbon and hydrogen with the oxygen of the air. 
The poor miners may be burned by the flame, or killed by 
the shock of the explosion ; and even if any of them escape 
these two causes of death, they are often asphyxiated by 
the carbonic acid gas which fills the galleries after the 



davy's safety lamp. 



i*3 



combustion. It is therefore necessary, above all, to ventilate 
the mines properly, to make considerable volumes of air cir- 
culate in the galleries, so as to carry the fire-damp away as 
fast as it is disengaged ; and lastly, to give the workmen a 
means of being warned of its 
presence as soon as there is 
any danger. 

To illustrate this, surround 
the flame of an oil-lamp (fig. 
40) with a covering of metallic 
gauze, and we shall see what 
happens when this lamp burns 
in the midst of the fire-damp. 
The combustible gas will enter 
the lamp through the meshes 
of the gauze, and will burn 
there by contact with the flame 
of the oil ; which will therefore 
become elongated and pale, 
and will entirely fill the interior 
of the network. But this flame 
will not be able to pass out, 
if the meshes of the gauze are 
sufficiently close, and if there 
are no holes formed by rough 
usage. The miner being warned 
by the appearance of the flame in his lamp, should, oi 
course, hasten to quit the gallery, but yet do so cautiously, 
without shaking the lamp, because it is still possible that 
little incandescent particles may escape through the gauze 
before it has had time to cool them. In this case the 
interior flame will extend outwards mechanically, and 
explosion w r ill take place. 

It is plain from this that scientifically, and indeed for all 
practical purposes, the problem how to secure safety to our 
miners has been satisfactorily solved. Unhappily, the em- 
ployment of Davy's lamp necessitates on the part of the 
men certain precautions, and these involve a certain amount 
of intelligence. When kept in bad repair it becomes a 

H 




Fig. 40. 
Sir Humphry Davy's Safety Lamp. 



114 THE PHENOMENA AND LAWS OF HEAT. 

source of danger. Some of the wires may be already partly 
destroyed by oxidation; the flame, enlarged by the fire-damp, 
then completes their destruction; the rotten gauze yields 
to the pressure, and explosion takes place. Thus accidents 
cannot always be avoided. These considerations have led 
to the invention of the electric lamp, burning in a vacuum 
tube, by which Davy's lamp may now be advantageously 
replaced. To describe it in this book would, however, be 
to depart from our proper task. 

2. Substances which are bad conductors. 

As yet we have occupied ourselves chiefly with solid 
bodies, which are good conductors of heat. Stones, glass, 
wood, and animal and vegetable tissues are bad con- 
ductors, through which heat is transmitted with difficulty. 
These substances will even completely arrest heat when in 
the state of powder or fibre, which is due partly to the fact 
that mechanical division destroys molecular continuity, which 
is necessary to conductibility, and partly that it interposes 
between the particles of the solid body layers of air, which 
conduct heat very badly. Thus, we can hold a red-hot 
cannon-ball with impunity, by covering the palm of the 
hand with asbestos. Artillerymen transport red-hot cannon- 
balls in wooden wheelbarrows filled with sand. Similarly, 
ice is preserved in sawdust; in the United States, ships 
are loaded with it in blocks of about two hundredweight, 
which are surrounded with sawdust, and thus sent into 
warm climates. In 1851 the export of ice amounted to 
50,000 tons, and the cost of sawdust for its transport 
amounted to nearly ^3 000. In spite of this precaution, a 
part of the ice melts during the voyage. In the passage 
from Boston to Calcutta four-fifths are melted in conse- 
quence of the long passage and great heat. 

In the construction of ice-houses the feeble conducti- 
bility of bricks is utilized. An ice-house is a deep pit lined 
with bricks (fig. 41), to prevent the heat of the soil from 
reaching the ice which is stored in them. The roof is 
covered over with straw, to intercept the heat of the sun, 
and trees are planted around, the foliage of which serves 



PRINCIPLE OF THE ICE-HOUSE. 



"5 



as an additional screen. Air must also be prevented from 
circulating in the interstices of the ice, to effect which the 
pit is filled up with water during the winter, so that by 
freezing it unites the pieces of ice into one solid mass. On 




Fig. 41. — Ice-house. 



the approach of warm weather, the ice gradually melts to 
some extent, and the water thus formed drains into a well, 
through a grating provided for the purpose, at the bottom of 
the pit. 

On account of their slight conductibility, bricks are used 
in northern countries for the construction of stoves. Ad- 
vantage is taken of their property of cooling slowly, after 
having been strongly heated. The fire is lighted only in 
the morning, and when all the fuel is transformed into hot 
coals, the openings are closed ; the heat is feebly radiated 



El6 THE PHENOMENA AND LAWS OF HEAT. 

by the exterior surface of the stove, which is covered with 
glazed delf, and it is sufficient to compensate the loss of 
heat by the walls of the room. Brick walls are also pre- 
ferable to those constructed of stone, which should be 
thicker, because the latter substance conducts heat better 
than brick. An excellent wall would consist of two wooden 
partitions filled with sawdust. It must be remarked that 
the same walls are good for hot countries as for cold ; be- 
cause by taking care to keep all openings closed during the 
day, they prevent the outer heat from entering. 

As an example of the singular facts explained by con- 
ductibility, Professor Tyndall mentions an instance of a 
steam-vessel being nearly lost under the circumstances men- 
tioned below. It happened, during a sea-voyage, that the 
engine-boiler became covered interiorly with a deep layer of 
earthy matters, deposited by the water. Deposits of this 
kind always occur on the sides of vessels in which ordinary 
water is boiled, the material being dissolved by the water 
from the earthy beds it has filtered through- In the above 
instance, however, the deposit had been allowed to collect 
beyond measure. Now this crust is a very bad conductor 
of heat ; the heat of the furnace therefore traversed it with 
difficulty; so that, to obtain sufficient steam to work the 
engine, it was necessary to burn more fuel than customary. 
The supply was exhausted before the ship had arrived in 
port ; they had then to burn the deck and all the wood that 
could be found in the vessel, The cause of this unpre- 
cedented consumption of fuel was discovered after arriving 
in port 

Tissues of organic origin are the worst conductors of 
heat. Many animals and vegetables are able to resist 
sudden changes of temperature, owing to the perfection of 
their natural clothing. We ourselves try to imitate nature 
in our apparel. Woollen stuffs prevent the body from losing 
its heat in winter, and from absorbing that of exterior bodies 
in summer, for which purposes it is more suitable than 
cotton, because it is a worse conductor than the latter. 
The material of our garments is always formed of sub- 
stances which have served to cover animals or vegetables. 



CONVECTION OF HEAT. 



117 



Warm-blooded animals, more especially, require consider- 
able protection, because the loss of heat suffered by a body 
increases with the excess of its temperature over that of 
surrounding space. Thus animals living in cold countries 
are covered with a thick fur, and the birds with a plumage 
even more efficacious than 
the fur. We find in these 
natural coverings the mecha- 
nical division of substances, 
also bad conductors, pushed 
to infinity. The thousands 
of filaments which constitute 
hair, or make up a feather, 
oppose a most effective ob- 
stacle to the transmission of 
heat. As to the aerial or aquatic 
animals, whose bodies have a 
temperature but slightly supe- 
rior to that of the medium they 
inhabit, they have no necessity 
for clothing ; the activity of 
their vital functions varies with 
the temperature of the medium 
in such a manner, that the tem- 
perature of their bodies suf- 
fers the same changes. These 
are the cold-blooded animals. 
Cold stupefies them, because 

their vital activity diminishes with the temperature ; warmth, 
on the contrary, reanimates them, because their vital activity 
augments under its influence. A direct relation is established 
between the interior and exterior conditions. 

Liquids conduct heat like solids ; but conduction is here 
habitually complicated with another phenomenon called 
convection. 




Fig. 42. — Convection of water. 



3. Convection of Heat in liquids and gases. 

We have here a glass vessel full of water, and heated 
from the bottom (fig. 42). A little sawdust is disseminated 



Il8 THE PHENOMENA AND LAWS OF HEAT. 

through the water, both having about the same density. 
The particles of wood may be seen to rise in the axis of 
the vessel, and to descend by the sides. What is the cause 
of this motion, or continuous circulation ? The water 
heated at the bottom becomes lighter ; it therefore rises, 
and with it the sawdust. As it rises, however, it is cooled, 
in some degree, by contact with the cooler layers through 
which it passes, and, arrived at the surface, it is still further 
cooled by contact with the air, and by radiation. The sides 
of the vessel are also cooled by the two last mentioned 
causes, so that the water which touches them is heavier 
than the water in the central parts. This heavier water 
falls to the bottom, where it is warmed, and from whence 
it rises up the centre, and so on as before. The particles 
of wood follow all the movements of the water, and serve 
to render them visible. 

Convection consists in this displacement of fluid strata 
thus unequally heated, and it results in the rapid diffusion of 
heat throughout the whole mass, although the liquid itself 
may be a very bad conductor ; for the warm parts heat 
the cold parts by contact with them, and this mingling of 
parts incessantly takes place so long as the vessel remains 
on the fire. If we wish to observe the conductibility only, 
we must prevent the transport of heat by convection ; and 
to effect this we have only to heat the liquid from above. 
Despretz accomplished this by placing on the surface of the 
liquid a metal box, through which a current of hot water 
was kept running (fig. 43), by which the liquid column was 
heated from above downwards. The first layer, on heating, 
became lighter ; it therefore remained on the surface : it 
heated the next layer lower down by conduction * but this, 
not being able to heat itself sufficiently to become lighter 
than the first, remained in its place, and in its turn heated a 
third layer: and thus the process went on to the end. 
Several thermometers disposed horizontally down the side 
of the vessel, as shown in the engraving, indicated, after a 
sufficient lapse of time, temperatures decreasing from above. 
Therefore, liquids conduct heat ; but they are in general slight 
conductors. Water, especially, is of very feeble conductibility ; 



CONDUCTIBILTTY OF LIQUIDS. II9 

though, for the want of sufficiently precise experiments, thii 
has been long denied. Thus, by putting water in a glass tube, 




Fig 43. — Apparatus to demonstrate the conduct bi'ity of liquids. 

with some ice at the bottom (fig. 44), the water may be made 
to boil without melting the ice. But this only proves that 
the water is of feeble conductibility, and not that it has 
none at all. 

It is much more difficult to prevent convection in gases 
than in liquids. Currents establish themselves not only in 
a vertical direction, in consequence of the unequal densities 
of hot and cold parts, but also in all other directions, in 
consequence of the expansibility of gases by heat. In all 
the experiments made on these fluids, the effects produced 
may be attributed either to conduction or convection. This, 
however, does not render it the less interesting to observe 
the effects produced by different gases. 

We observed in Chapter III. that the metallic, wire 



120 



THE PHENOMENA AND LAWS OF HEAT. 



uniting the copper and zinc at the two ends of a Voltaic 
pile was heated by the interior motion emanating from the 
pile, which is called the electric current. If the wire be 
replaced by a series of metallic parts, among which is a 
platinum wire of sufficient fineness, this last may be heated 




Fig. 44. — Water boiling over ice. 

to redness. For example, a wide glass tube has the wire 
fixed in its axis, with the help of copper wire traversing 
the corks adapted to the extremities of the tube (fig. 45). 
Each of the corks carries besides a smaller glass tube. 
These tubes, being open, the air naturally fills the appa- 
ratus, envelopes the fine wire, and cools it. This is proved 
by the fact that, if we close one of the small tubes, and 
remove the air by the other with an air-pump, we shall see 
the incandescence of the wire increased. 



CONDUCTIBILITY OF GASES. 



T2I 



Now open the little tubes again, and 
adapt to one of them a bladder filled 
■vith hydrogen gas ; by pressing the blad- 
der, we shall cause the gas to enter the 
tube, and thus surround the wire ; we 
shall immediately see the wire lose its 
incandescence. Therefore, hot bodies are 
cooled in a greater degree by the contact 
of hydrogen than by that of air. 

It is supposed that conductibility is in 
the above experiment the principal cause 
of the difference observed in the beha- 
viour of the air and the hydrogen, and 
that the latter is the best conductor of all 
the gases. The effect of the hydrogen 
gas in contact with the incandescent wire 
was the same as if the latter had been in 
contact throughout its entire length with a 
metal 

When convection is reduced to its least 
possible activity, the air transmits heat 
very badly, from which it of course follows 
that air is but a bad conductor of heat. 
This property of the atmosphere contri- 
butes to the efficacy of furs and other 
articles of clothing. A stagnant bed of 
air fills the interstices of their filaments, 
and arrests the heat. If they were com- 
pressed strongly so as to remove the air, 
they would become better conductors. If 
this air were not stationary, if it were 
subject to renewal, it would transmit heat 
by convection, and the clothing would 
thus lose its efficacy. This explains why 
furs are warmer when the hair is turned Y Vv Jj 

inwards. \. >7*£i 

The double windows used in cold 
countries, and in our hothouses, are an fig. 45— Apparatus foi 
application of these principles. Between ^o^of^Is^ 



122 THE PHENOMENA AND LAWS OF HEAT. 

the two panes of glass there is a thin stratum of air, which 
cannot renew itself, and in which the currents are very 
feeble ; it acts therefore like a bad conductor, preventing 
*he heat of the room or hothouse from escaping by con- 
duction. Neither is the heat transmitted by radiation, 
because glass does not allow the dark rays to pass. As to 
the solar rays, which are of another nature, they enter 
freely, and contribute towards the elevation of the interior 
temperature. 

The celebrated Saussure constructed a wooden box, 
blackened interiorly, one of its sides being formed of three 
panes of glass separated by thin layers of air. By putting 
a vessel of water in the box, and exposing the glass side to 
the rays of the sun, he was able to make the water boil 
The explanation of this curious experiment forms a recapitu- 
lation of what we have already stated concerning radiation 
and conductibility. The heat of the sun traverses the glass 
and the interposed layers of air by radiation, and is readily 
absorbed by the blackened sides. Being transformed into 
dark heat, it can no longer traverse the glass by radiation ; 
it is therefore entirely employed in heating the air and 
water contained in the box. The sides of the box, strongly 
heated interiorly, keep at a high temperature, in consequence 
of their slight conductibility, which renders them insensible 
to the cooling action of surrounding bodies. Finally, the 
glass surface acts in the same way, in consequence of the 
two layers of air which remain stationary. 

4. Effects of convection in the ocean. — Marine currents. 

Conductibility plays an important part in the physics 
of the globe. The distribution of the solar heat on the 
surface of the earth is more especially effected by convec- 
tion in the waters of the ocean and in the atmosphere. 
Immense currents form in the midst of the seas, and give 
rise to certain laws of temperature, which are as yet but 
obscurely known. Other currents agitate the strata of the 
atmosphere at various heights ; and these constitute the 
winds which sweep over our continents, and bring in turn 



MARINE CURRENTS. 



123 



dryness and humidity, in obedience to laws of which again 
a few only have been observed. 

To understand the manner in which heat determines the 
marine currents, imagine the terrestrial globe to be entirely 
surrounded with water, and consider the action of the solar 
rays on that immense ocean. At the equator, these rays 
arrive in a direction but little removed from the vertical; as 
we near the poles, they become farther removed from this 
direction ; and at and close to the poles, they glance across 



nearly horizontal direction. 




the surface of the earth in a 
Their heating effect, therefore, 
diminishes from the equator 
towards the poles. In con- 
sequence, the superficial layers 
of water in the equatorial 
regions have a more elevated 
temperature than those of 
the polar regions. The latter 
therefore descend to the bot- 
tom of the ocean, and form 
lower currents, which travel 
from the poles towards the 
equator, and become progres- 
sively heated. On reaching 

the equator, these masses Of FlG - 4 6 *— Theory of Marine Currents. 

heated water rise to the surface, 

and there acquiring a higher temperature, they return 
towards the poles in the form of upper currents. Thus an 
incessant circulation would occur in the liquid envelope of 
the globe, as indicated by the arrows in fig. 46. 

Now consider the earth as it exists, with its several 
continents and seas. The form and temperature of the 
coasts modify the currents which should be established 
between the poles and the equator. In seas which are 
nearly land-bound these currents cannot take place, and 
the local action of the solar rays alone regulates their tem- 
perature : thus the water of the Mediterranean is warmer 
than that of the ocean ; the waters of the Indian sea, not 
having any issue in their northern parts towards the North 



124 THE PHENOMENA AND LAWS OF HEAT. 

Pole, get considerably warmed, and contribute to intensify 
the summer heat in the neighbouring countries. But in the 
Atlantic and Pacific oceans, which stretch from one poie to 
the other, marine currents can be formed, and their existence 
has been proved by travellers. Admiral Duperrey and 
Captain Maury have made a special study of the subject, 
and their discoveries have led to a very important applica- 
tion of scientific knowledge. A ship, for example, designed 
to sail from America to Europe, has only to be directed into 
a current flowing in this direction between the two conti- 
nents, and it will effect the passage in a much shorter time 
than if it took another route. 

Those marine currents which are perfectly known are 
shown in the annexed chart (fig. 47). The one most impor- 
tant to us, in consequence of its influence on the climate 
of Western Europe, is the Gulf-stream, which carries 
towards the coasts of England, France, and Spain consi- 
derable quantities of water heated under the burning sun of 
Central America. To understand this current, the arrows 
traced on the map must be carefully followed. The eye is 
at once arrested by two immense whirls in the Atlantic 
Ocean ; one of them situated below the equator, the other 
above. The first is formed by a current coming from the South 
Pole to cool the western coast of Africa, and, flowing as far 
as the equator, it there acquires an elevated temperature, 
and divides into two branches. One of these descends by 
the coasts of Patagonia, to complete the circuit ; the other 
rises along the coast of Chili, to enter the second whirl, 
and, striking off from the Gulf of Mexico, takes the name 
of the Gulf-stream. It leaves the Gulf at a speed of about 
four and a half miles per hour, and a temperature of about 
8o° Fahrenheit. It first travels northward, then towards 
Newfoundland, and, turning sharply to the east, it forms 
two branches. The descending branch bathes the shores 
of England, where it maintains a moderate temperature, 
and returns, by a route not very distant from the French 
and Spanish coasts, to the equatorial regions, where the 
circuit is completed. The second branch flows northward, 
tempering in its course the climates of Ireland and Norway. 



i 




u 



S 



THE GULF-STREAM. 1 27 

The complete circulation of the Gulf-stream may be de- 
scribed as a voyage of more than three thousand leagues, 
and occupying three years. Its temperature rises to 82^° 
Fahrenheit in the neighbourhood of the equator, and oppo- 
site the coasts of the United States is still 64^° ; whilst at 
the same distance from the equator, but out of the current, 
the water of the ocean is only 57 Fahrenheit. It is easy 
to conceive the variety of effects that such currents might 
produce. Those which come from the poles carry icebergs 
with them, which melt on approaching the warm latitudes. 
In other places, trees and debris of all kinds are transported 
from one continent to another; seeds and eggs will thus 
travel long distances, and afterwards germinate or be 
hatched, thus explaining some of the most curious facts as 
to the migration of species. Even man himself finds his 
road traced on the ocean ; and it is sufficient to glance at 
the chart to be convinced that the nations of the South 
Sea Islands were never able to reach America with their 
pirogues, an immense current flowing across the Pacific 
Ocean from the east to the west constituting an insur- 
mountable obstacle for such vessels. Without stopping to 
describe all these effects, we must now give our attention to 
the consideration of the distribution of heat. Peru, for 
example, and Brazil are very differently situated with regard 
to marine currents. The first is subjected to the cooling 
action of a current from the South Pole ; also, although near 
the equator, it enjoys on the sea-shore a moderate tempera- 
ture ; the inhabitants are able to cultivate the soil without 
employing slaves, and a corresponding softness of manners 
is the result. In the Brazils, on the contrary, subject to 
the action of the equatorial current of the Atlantic, the 
excessive heat has caused the Portuguese to employ African 
slaves in the labours of cultivation. 

Marine currents do not depend only upon the action of 
the sun's rays on the water ; they are also intimately related 
to the winds, the diurnal rotation of the earth, and various 
other circumstances which render it very difficult to specify 
precisely the laws which regulate them. This study is never- 
theless sufficiently advanced to have enabled navigators 



128 THE PHENOMENA AND LAWS OF HEAT. 

to make brilliant geographical discoveries. Doubtless, 
there is in our day no new world to discover; man has 
entirely overrun the surface of the earth, but the mystery 
of the polar regions remains to be penetrated. Hardy 
seamen have often visited the arctic seas, in the hope of 
discovering some new land, and death has often put an end 
to their perils and sufferings. Still, the interesting problem 
remains unsolved. Does the ice extend to the poles them- 
selves ? Are we to believe that in these immense regions 
nature wears an aspect of perpetual horror, and that no 
animated being can be sustained in existence ? 

Let us try to answer this important question. Travellers 
assure us that near the poles marine currents travel towards 
the equator on the surface of the ocean ; it is these which 
carry the icebergs. We shall see, in the next chapter, why 
these ice-bearing currents of water are surface ones; but 
it follows that the water which leaves the poles must be 
replaced by warmer water coming from the equator, and, 
therefore, there should be in the depths of the frozen seas 
immense sub-marine currents carrying heat towards the 
poles. It is by no means impossible, therefore, that the 
polar regions may consist of habitable seas, surrounded on 
every side by a circle of eternal ice ; this ice being melted 
below and renewed above by the condensation of the 
vapours from the seas they surround, there would be a per- 
petual compensation between these two inverse effects, and 
a law of equilibrium according with that which regulates the 
currents of the equatorial regions. The distribution of heat 
in the terrestrial waters would in this case be effected by 
two systems of circulation, as indicated in fig. 48. 

This theoretical hypothesis was verified in 1853 . after two 
years of infinite fatigue, in the midst of the greatest perils, 
Dr Kane, of Philadelphia, discovered near the North Pole 
a sea inhabited by animals such as those ordinarily met 
with in temperate regions. A thick mist entirely covered 
it, the result of atmospheric cold and the warmth of the 
waters. Unhappily, the exploration was brought to an 
abrupt termination. The exhausted voyagers lived long 
enough to reach their country, and Kane died in 1857, from 



CONVECTION IN THE ATMOSPHERE. 



129 



die effects of his protracted sufferings, after ha\ing made 
his disanery known, and informed geographers of the 
route they must follow to complete it. 




Fig. 48. — Theory of Polar Currents. 

5. Effects of convection in the atmosphere. — Winds. 

It remains for us to examine the effects of the convection 
of heat in the atmosphere, effects which are more complex, 
perhaps, than those of the ocean, but from which it is, never- 
theless, possible to discover a general law. 

To take the most simple case, let us imagine ourselves 
on the sea-coast after a fine day. Night arrives, and the 
warmth of the sun is gone. Radiation towards the celestial 
space succeeds to the absorption of the solar rays. In the 
course of the day the sea has absorbed more heat than the 
soil, and, besides, the heat has penetrated into its depths. 
During the night the sea will cool more slowly than the 
earth, because the lower strata will continually replace the 
upper, subject to nocturnal radiation. The air situated over 
the sea will still further abate its cooling, because it is 
moist, and the vapour of water absorbs the dark rays of 
heat. The air will, therefore, be warmer than that which 
covers the earth : it will receive heat from the sea by con- 
duction and by absorption. Now, hot moist air is lighter 
than cold dry air ; the air over the sea will therefore rise, 
and the air over the land will descend in order to occupy 
its place, slipping along the surface of the shore towards the 

i 



130 THE PHENOMENA AND LAWS OF HEAT. 

sea. This causes a wind called the evening breeze, which 
lasts as long as there is sufficient difference in the tempera- 
ture of the land and the sea, and so long as the air is not 
disturbed by winds due to other causes. It is this evening 
breeze which the sailor utilizes in leaving a port. 

In the morning, when the sun again shines, the same 
phenomenon occurs, but inversed. The earth gets warm 
quicker than the sea, and the breeze comes from the sea 
towards the land : it brings ships into port. 

In the same manner mountain breezes may be explained, 
In the evening we feel in the valley a breeze coming from 
the tops of the mountains, because there the cooling is 
most intense. In the morning the wind changes in direc- 
tion, because the summit of the mountain receives the sun's 
heat before the bottom of the valley. 

Extend our reasoning to large surfaces of country and 
great masses of air, and we have the explanation of the 
trade-winds (monsoons), which blow in some countries for 
six months in one direction, and for six months in the 
opposite. In the Mediterranean, for example, the wind 
as a rule comes from the north during summer, and from 
the south during the winter. We pass over the accidental 
winds by which the atmosphere is at times disturbed. The 
summer monsoon is produced by the heating of the Sahara 
desert, and that of the winter by its cooling; whilst the 
sea and the southern coasts of Europe preserve a nearly 
constant temperature. The inequality in the length of the 
passage between Toulon and Algiers, which is shorter in 
one direction than in the other, is due to these monsoons. 
As the heating of the desert is more intense than its 
cooling, the monsoon from the north is stronger than that 
from the south, and a sailing vessel which makes the pas- 
sage regularly in both directions during a whole year takes, 
on an average, a longer time to accomplish the voyage from 
Algiers to Toulon than that in the reverse direction. 

It will assist our application of these facts to the entire 
earth if we imagine, as we did in our study of the marine 
currents, the whole surface of the globe to consist of water, 
and this again to be surrounded by a stratum of air. It is 



CAUSES OF WINDS. 131 

At the equator, as before, that the liquid surface is most 
heated. The air at the equator will, therefore, be lightest, 
in consequence both of its temperature and of its humidity, 
which rises with its temperature. If the earth were im- 
movable, this air in rising from the surface would be replaced 
by colder air coming from the poles, and it would establish a 
circulation like that represented in fig. 46. In the northern 
hemisphere there would be a perpetual north wind, and in 
the southern a perpetual south wind ; the two winds meeting 
at the equator with an equal but contrary motion, would 
there destroy each other, and there would be a region of 
perpetual calm the whole length of the equator. 

But the earth turns on its axis in one day. Its speed at 
different points of its surface decreases from the equator to 
the poles, where it is null. Therefore, as a mass of polar air 
descends towards the equator, it will pass over a surface 
which goes quicker than itself towards the east. Let us 
consider the effects of this. 

Suppose that we are stationed on this surface ; it would 
carry us towards the east, and the air which surrounds us 
would also move in the same direction, but less quickly than 
ourselves. We should, therefore, traverse and displace it, 
and the resistance which it would oppose to us would be 
exactly the same as that of a wind coming from the east. 
Further, if the air possessed at the same time this movement 
towards the east, less rapid than ours, and a movement from 
the north pole towards the equator, its resistance would be 
that of a north-east wind. 

In this manner the diurnal rotation of the earth modifies 
the circulatory movement of the masses of atmospheric air, 
due to the convection of heat. In the northern hemisphere 
we should have a north-east wind ; in the southern hemi- 
sphere a south-east wind, for a similar reason \ and along 
the equator an east wind, resulting from the combination of 
the two preceding. 

In place of the imaginary liquid earth, let us now con- 
sider the earth as it exists with its continents and its seas, 
the latter, in the equatorial region, covering the greater part 
of its surface. The same cause for winds will continue to 



I32 THE PHENOMENA AND LAWS OF HEAT. 

exist ; but a crowd of complications will result from the in 
equality of the surface, and the winds indicated by our reason- 
ing will not always exist. They are called trade winds. The} 
w r ere observed for the first time by Christopher Columbus, 
and carried his ship towards America. It is related that 
his companions were terrified by them, because they feared 
they should be unable to retrace their steps. We owe the 
first explanation of these winds to the illustrious Halley. 

While the trade winds can be proved to exist on the 
surface of the globe, it appears more difficult to verify the 
existence of upper currents, which, predominating in the 
higher regions of the atmosphere from the equator to the 
poles, complete the circulatory movement. Their direction 
should be the reverse of the lower regions ; that is to say, 
south-west in the northern hemisphere, and north-west in the 
southern. The existence of these winds is proved, however, 
by the pulverulent matters which they carry from one region 
to another, and of which the following is a curious example. 

In the spring and autumn, a dust may be often collected 
in France and Italy, coming in the form of a shower, and 
in which the microscope reveals organic remains from 
Central America. There are marshes which are dried up 
at these periods, and which are swept by very violent 
whirlwinds. They raise the dust of the soil to the height 
of the upper trade-wind, which, coming from the south-w r est, 
transports it towards the north-east, into Europe, in the 
space of about a month. 

We may cite further the case in which ashes and cinders 
were transported from a volcano in Guatemala, which 
happened in 1853. The ashes were blown from the west 
eastwards, towards Jamaica, and were so abundant, that the 
country was darkened by them for several days. 

From all these observations we conclude, that if the con- 
vection of heat in the fluid part of our globe is not the only 
cause of marine and atmospheric currents, it is at least their 
principal cause. It is only after having thoroughly defined 
the manner in which this cause acts, that we should occupy 
ourselves with the consideration of other causes. We then 
enter the domain of meteorology. 



CHAPTER VI. 

CHANGE OF THE VOLUME OF BODIES. 

I. Action of Beat on gases. — Sensible Heat — Exterior work. 

Place a well-closed bladder nearly filled with air near the 
fire ; it will gradually expand, and at length burst. And 
yet the quantity of air it contains remains the same, because 
communication is not possible between the interior of the 
bladder and the atmosphere. At the same time the bladder 
becomes somewhat heated, and if we tie its neck round the 
tube of a thermometer (fig. 49), placing the bulb in the centre, 
we shall see that the temperature of the interior air rises. 
Remove the bladder from the fire ; it will contract of itself, 
and, growing cool, will resume its former condition. Thus, 
in absorbing heat, the air in the bladder has undergone two 
modifications, an elevation in temperature and an augmenta- 
tion in volume. In losing heat it, on the contrary, suffers a 
diminution both in temperature and volume. The change 
of temperature is easily understood : it is neither more nor 
less than the manifestation of the property possessed by 
all bodies of being heated or cooled, of changing their 
calorific state by absorbing or disengaging sensible heat, and 
this term we shall always employ to designate heat which 
is appreciable by the thermometer. But the change of 
volume is a different matter, which we shall now proceed to 
consider. 

The bladder is pressed interiorly by the air it contains, 
and exteriorly by the atmosphere, and as it is very flexible, 
and not elastic, we must admit that the two pressures are 
in equilibrium. When it is heated, the interior pressure 



134 



THE PHENOMENA AND LAWS OF HEAT. 



increases a little : the bladder swells until the equilibrium 
between the two pressures is re-established, and if the 
heating is continued, these variations succeed each other 
gradually, without the interior pressure sen- 
sibly exceeding that of the atmosphere, 
supposing that the bladder is veiy flexible 
and not fully expanded. The air in the 
bladder is said to be heated under a con- 
stant pressure equal to the pressure of the 
atmosphere. Now, this gradual expansion 
of the bladder exercises a pressure against 
the atmosphere ; each portion of its surface 
equal to a square centimetre receives a 
pressure of about one kilogramme.* If it 
is displaced (swollen by expansion) one 
centimetre in extent, a mechanical work is 
effected equal to that produced by lifting a 
weight of one kilogramme one centimetre 
in height. The displacement of the atmo- 
sphere effected by the whole surface of the 
bladder in expanding is therefore a mecha- 
Expansion of nical effect, measurable in kilogrammetres, 
and, from the principle of the equivalence 
of heat and work established in our first 
chapter, this production of work corresponds to the expen- 
diture of a certain quantity of heat. 

From these facts we deduce a first rule : When a gas is 
heated whilst under a constant pressure, a part of the heat 
which it receives remains in the gas in a sensible state, and 
serves to raise its temperature ; another part is really con- 
sumed, loses its force as heat, and is transformed into 
mechanical work. 

As the molecules of a gas are not mutually attracted, 
their separation is effected without expenditure of appre- 
ciable force. We have not, therefore, to seek in these 
bodies a third method of the action of heat. 

The cooling of the bladder is an inverse operation to the 
preceding ; the sensible heat of the gas passes into neigh- 
* Estimated in English at 15 lbs. to the square foot. 




Fig. 49 
air under constant 
pressure 



ACTION OF HEAT ON GASES. 



*35 



bouring bodies, without changing its state. In bringing the 
bladder back to its primitive form, the atmosphere acts as a 
compressing force, and a mechanical work is expended 
which is transformed into heat ; this heat, like the sensible 
heat, passes into surrounding bodies. 

We have an example of the heating of a gas under a 
constant pressure, equal to that of the atmosphere, in 
fire-balloon?. The first experiment was made by Joseph and 




Fig. 50.— Montgolfier's Balloon. 



Stephen Montgolfier, at Avignon in France, in December 
1782. These brothers made their balloon of linen lined 
with paper, and having a diameter of thirty-five feet An 



136 THE PHENOMENA AND LAWS OF HEAT. 

opening was made at the bottom, so that, by lighting a large 
fire underneath, the balloon was filled with hot air, and 
made to rise with a force of 500 kilogrammes. In a second 
experiment made at Annonay, in the following year, a basket 
made of iron wire was suspended under the opening of the 
balloon, and the fire was placed in it, so as to be maintained 
during the ascent. The balloon was thus raised 2000 metres. 
A larger balloon was made to ascend at Versailles, before the 
court, and on this occasion it carried a cage containing 
some animals, which came to the ground safe and sound. 
After these first trials, two men had the courage to venture 
into the hitherto unexplored regions of the atmosphere, and 
created a new art, that of aerial navigation. These were 
the Marquis d'Arlandes and'Pilatre de Rozier, who, on the 
20th of November 1783, left the Chateau de la Muette, 
and first hovered over Paris in a balloon (fig. 50). Their 
success encouraged similar enterprises in many other places ; 
hydrogen gas was substituted for hot air, and the confidence 
became so great, that Pilatre de Rozier and Romain 
attempted to make an aerial passage across the Channel. 
Unfortunately, they conceived the idea of uniting two 
balloons, the one filled with hydrogen, the other with hot 
air, and of attaching the last-named under the first. They 
had scarcely left Boulogne, when the machine caught fire, in 
consequence of the combustibility of the hydrogen gas, and 
was destroyed in an instant, the voyagers being thrown head- 
long on to the shore. Their tomb may be seen at Vimille, a 
village near Boulogne. This catastrophe did not prevent the 
number of aeronauts from increasing ; and now balloons are 
often employed, both for exhibition and for scientific excur- 
sions, or for the purpose of making observations in time of 
war. Latterly, Paris has witnessed the ascent of the largest 
balloon ever made, that called the Aigle; but it was so 
difficult to handle, that a few trials only have been made 
with it. 

By what force do balloons rise ? Is it heat ? Evidently 
not, because they may be filled with hydrogen, which 
remains cold. But heat causes the air they contain to 
become lighter, and then the pressure of the surrounding 



ACTION OF HEAT ON GASES. 1 37 

air makes them rise. The balloon ready to rise is in the 
midst of the surrounding air like a cork held beneath the 
surface of the water. As soon as the cork is set free, it 
rises to the surface. An analogous phenomenon occurs daily 
in our chimneys, and produces the current of air generally 
called the draught. The gases which fill the chimney are 
dilated by heat, and they weigh less than the same volume 
of cold air ; the latter therefore descend on all sides, and 
force the hot air to rise. This cold air should enter by 
openings in the room; and if these are not sufficient, it will 
descend by the chimney itself, by the side of the hot gases 
which are ascending, and thus two contrary currents will be 
established. The descending current will of course drag 
some smoke down with it, which will therefore enter the 
room. It is to avoid this that air-holes are disposed around 
the grate, by which the exterior air is introduced, without 
depending on the doors and windows. 

The various causes of the displacement of strata which 
came under our notice in the preceding chapter, whether 
occurring in the ocean or in the atmosphere, and resulting 
in the distribution of heat by convection, are due to the 
same cause, namely, expansion or dilatation by heat. 

A gas may be so heated, that none of the heat is employed 
to effect mechanical work. This is done by placing the 
gas in a strong reservoir, so that its volume may remain con- 
stant whilst being heated. It is, in point of fact, impossible 
to heat a gas in a reservoir without also heating the latter, 
and thus changing its volume ; but the expansion produced 
by heat in a solid reservoir is so slight, that it may be dis- 
regarded in the experiment. An arrangement by which the 
experiment can be tried is represented in fig. 51. A large 
glass flask containing air, and a shallow layer of a non- 
volatile liquid, such as oil, is closed by a cork fitted with a 
narrow glass tube and a thermometer. The glass tube is 
vertical, and its extremity is plunged into the liquid. When 
the flask is heated, the effect on the thermometer may 
immediately be noted, and at the same time the oil will be 
seen to rise in the tube. We conclude from this observa- 
tion that the elastic force of the air contained in the flask 



*3* 



THE PHENOMENA AND LAWS OF HEAT. 



is augmented at the same time as its sensible heat; and this 
augmentation may be measured by the height to which the 
liquid column is raised. As to the bulk of this air, it is 
increased to an extent equal to the volume of the oil raised 
in the tube, and to that which corresponds to the expansion 

of the flask. We have therefore 
demonstrated the following property 
of gases : When a gas is heated with- 
out its volume being allowed to 
change, its elastic force increases 
at the same time as its temperature. 
Generally, this increase of elastic 
force is not so visible as in the pre- 
ceding experiment. Take, for ex- 
ample, the apparatus represented by 
figure i, which consists of a hollow 
copper ball, mounted on wheels. 
Stop the opening tightly, and light 
the spirit-lamp beneath. The air in 
the ball will now be heated, whilst 
its volume is as nearly as possible 
unaltered. It presses the sides of 
its prison at all points ; but they 
resist, and nothing makes this pres- 
sure visible. As soon as the tem- 
perature rises to about 272 , each 
square centimetre of the internal 
surface will be subjected to a pres- 
sure from within equal to a force 
of more than two kilogrammes, 
whilst on an equal extent of the 
outside surface the atmosphere exercises a pressure of only 
one kilogramme. If the stopper exhibit a surface of one 
square centimetre, it will therefore be now submitted to a 
pressure of one kilogramme from within ; and in conse- 
quence, if it is not firmly fixed in the orifice, it will be 
thrown out, and the apparatus will recoil, as already ex- 
plained. If the stopper be fixed more firmly, the temperature 
must be raised higher ; the elastic force will increase, and 




Fig. 51. — Heating of a gas, its 
volume remaining the same. 



ACTION OF HEAT ON GASES. I39 

may become sufficiently great to make the stopper fly out 
We shall have a proof of this increase in the force of the 
recoil, which becomes greater as the temperature rises 
higher at which the explosion occurs. 

This experiment explains explosions that take place under 
a number of circumstances. In most cases these are due to 
inflammable substances being contained in a closed space. 
The combustible takes fire, accompanied by chemical com- 
bination, and the disengagement of heat and light. The 
mixture becomes intensely hot, and there is no room for it 
to expand. Its elastic force increases rapidly, until at last it 
overcomes the obstacles opposed to it. When this takes 
place, the gases rush out with violence • and the disturbance 
thus occasioned is the cause of the noise we hear. 

Explosion may also take place without the intervention of 
the air; for example, with gunpowder, because the substances 
which compose it become transformed into gas, as we have 
seen in the first chapter. 

When a gas is heated, its volume being kept constantly 
the same by confinement in a close place, the heat is only 
employed to give its molecules a particular activity, which 
is manifested by an increase of elastic force and sensible 
heat \ it must not be supposed that the increase in elastic 
force is a mechanical work which consumes heat. The 
heat produces only a single effect, which is elevation of 
temperature, and there is no work done, because, in the 
modification which the gas undergoes, no resistance is over- 
come by force. To say that the heated gas presses the 
sides of the vessel more than the cold gas, is to say that 
the heat it acquires augments its expansibility ; there is no 
resistance overcome here, as in the case of the gas being 
heated under a constant pressure. The sensible heat sup- 
plied to the gas gives it an expansive property, which would 
serve to measure this heat, as well as its action on the 
thermometer. It may be said that expansibility and tem- 
perature in a gas are two forms of the sensible heat which 
it contains. 

We therefore arrive at this second rule relative to the 
heating of gases : When a gas is heated, its volume remaining 



I40 THE PHENOMENA AND LAWS OF HEAT. 

constant, all the heat it receives remains in the gas in a 
sensible state, and serves to elevate its temperature : this 
elevation in temperature is accompanied by an increase 
in the pressure against the sides of the enclosing vessel, 
and is neutralized by their resistance. Reciprocally, when a 
gas is cooled, its volume remaining constant, it loses a part 
of its sensible heat, which passes into exterior substances, 
the temperature and pressure falling simultaneously. 

From our two rules we may draw the following conse- 
quence. To raise the temperature of a kilogramme of air 
from zero to 272 degrees, it would be necessary to consume 
a larger quantity of heat, if the air dilated whilst preserving 
the same pressure or elastic force, than if it were kept at 
a constant volume ; because in the first case heat is expended 
to effect a mechanical work which does not take place in the 
second. This conclusion is confirmed, in fact, by abundant 
experience. 

Gases are most often heated under such circumstances 
that there is a simultaneous change in volume and pressure. 
The quantity of heat which they consume then depends 
both on the elevation in their temperature, and on the 
mechanical work put in play. Everything else being equal, 
it therefore varies with this work. A kilogramme of gas 
consumes when rising in temperature, to a certain extent, and 
reciprocally disengages, when falling, different quantities of 
heat, according to the mechanical effects which take place. 
When the volume of the gas augments, a mechanical work 
is produced, and a certain amount of heat disappears ; 
when it diminishes, the mechanical work is spent, and heat 
appears. We have discussed the relation between heat and 
work whilst occupied with the mechanical equivalent of 
heat. 

The principle enunciated above explains how the hot-air 
engine produces work by the consumption of heat Whilst 
dilating, the air contained in the machine produces work ; 
whilst contracting, the same air expends work, but less in 
amount, the difference between these two being the part 
utilized. Further, in the former case heat is consumed, and 
in the latter a less quantity is disengaged ; the difference 



ACTION OF HEAT ON SOLIDS. 141 

between these two quantities of heat is destroyed, or rather 
converted into work. 



2. Action of Heat on solids and liquids, — Grandeur of 
molecular forces. — Interior work. 

The action of heat on solids and liquids is more compli- 
cated than its action on gases, because the molecules are 
held together with considerable force. Imagine a mass of 
iron heated in the midst of the atmosphere. It expands 
and produces a preliminary mechanical work analogous to 
that of the vessel filled with air which was under considera- 
tion at the commencement of the present chapter. A cubic 
decimetre of this substance, heated from o° to ioo° C, in- 
creases in volume scarcely four cubic centimetres : if this 
mass of iron have the cubic forn, each edge lengthens about 
twelve-hundredths of a millimetre. The atmosphere exerts 
over each of the six faces of the cube, a pressure of 103 
kilogrammes, and this is the measure of the exterior resist- 
ance which the heat must surmount in order to expand the 
body. The total work produced is the displacement, by 
six-hundredths of a millimetre, of six times 103 kilogrammes, 
a quantity less than one-tenth of a kilogramme. As we 
know that each thermal unit is equal to 425 kiiogrammetres, 
we may conclude that the heat consumed by overcoming 
the resistance of the atmosphere in the phenomenon under 
consideration is very small, and need not be taken into 
account. But we shall find a second kind of work which 
we did not meet with in gases. 

The molecular forces in solids and liquids may, without 
exaggeration, be called stupendous. Experiments made 
on the elasticity of solid substances show that, to lengthen 
by twelve-hundredths of a millimetre the side of a cube of 
iron one decimetre in length, requires a force of about 
250,000 kilogrammes. Such would be the exterior force 
necessary to overcome the resistance of the molecular forces. 

Now that which requires this enormous mechanical effort 
heat effects naturally, and at the same time raises the tem- 
perature of the cube of iron from o° to ioo°C. It therefore 



142 THE PHENOMENA AND LAWS OF HEAT. 

overcomes the resistance of the molecules of iron, and 
separates them after the manner of a veritable force, thus 
effecting an interior work capable of being expressed in 
kilogramme tres. Each time 425 kilogrammetres are thus 
produced in the interior of a body, one thermal unit will 
have been expended, for which we shall not be able to 
account in the amount of sensible heat possessed by the 
body. 

We have another proof of the grandeur of the molecular 
forces in the enormous mechanical efforts developed by the 
expansion and contraction of solid and liquid substances 
when they undergo a change in temperature. 

In making a cart-wheel, the workman heats the iron 
hoops so as to expand them, when they pass over the 
wheel very easily, and are allowed to cool : the contraction 
of the tire causes it to bind the wheel with great force. 

The iron rails laid on railways are not fitted closely end 
to end, but are always separated from each other by a 
small interval, so that they may be able to expand freely in 
their length. Without this precaution they would bend 
during the heats of summer. In fact, if the rails were close 
together over a length of 250 miles, the total change in 
length, from winter to summer, would be over 900 feet; 
and, as the sleepers fix them firmly to the soil, they could 
not suffer such a change without twisting out of shape. 

Again, it is the force of expansion or contraction which 
causes the rupture of substances which are bad conductors 
of heat, when they are subjected to a sudden change of 
temperature. For example, if you touch glass with a piece 
of red-hot iron, the glass breaks. It is because the parts 
touched expand rapidly, and the neighbouring parts remain- 
ing cold obstruct the expansion ; they are roughly repulsed 
in every direction, so to speak, and separate from each 
other. The same effect would take place if the glass were 
touched by a substance excessively cold ; but in this case 
the rupture would be due to the rapid contraction of the 
molecules. The possibility of breaking cold glass by the 
contact of a very hot substance is utilized in cutting glass 
vessels. A mark is made on the glass with a file, and a 



ACTION OF HEAT ON SOLIDS. 1 43 

piece of red-hot charcoal applied to it ; the rupture takes 
place in the direction of the mark ; the charcoal being then 
drawn along just in advance of the crack, and in the 
necessary direction : and the glass may be cut in this direc- 
tion if it has no irregularities of structure. 

To observe the force of expansion in liquids, a vessel 
made of glass, or other resistant material, may be filled 
with the liquid upon which we wish to operate, closed 
hermetically and heated ; the vessel will be broken if the 
cork be quite tight. 

Take an iron vessel, having somewhat the shape of a 
bottle or flask, and holding about a quart ; fill it with water 
and close it with a screw stopper, then heat it to boiling 
point. The expansion of the iron vessel will barely amount 
to a quarter of a cubic inch, whilst the water will increase in 
bulk more than two and a half cubic inches, about ten times 
more than the vessel. This water will therefore exercise an 
enormous pressure on the iron which encloses it, distort- 
ing its shape, and finally breaking it, if it be not sufficiently 
strong. 

A very curious application of the force of contraction ot 
solids has been made by the architect Molard, on the build- 
ing of the Conservatoire des Arts et Metiers in Paris. The 
walls of a vaulted gallery had been pushed outwards by the 
weight of the superincumbent masonry, and it was feared 
the whole would fall. Molard arranged iron bars in a 
parallel direction, passing through the walls, and carrying at 
both ends a screw thread fitted with screws (fig. 52). He 
heated the bars throughout their whole length, and having im- 
mediately screwed them up tight, allowed them to cool. The 
contraction of the iron, which proceeded slowly as it cooled, 
drew the walls nearer together without endangering them; 
and this was repeated several times, until the walls were 
re-established in a vertical position. The bars were left to 
maintain them so, and may be seen to the present day. 

The laws of the dilatation of metals deserve the more 
careful study, seeing that iron especially is now so generally 
used for building purposes. A serious question might be 
raised, whether its universal employment is not in some 



144 



THE PHENOMENA AND LAWS OF HEAT. 



measure censurable ; whether, in fact, our houses and public 
buildings are not exposed to accidents owing to simple 
change of temperature. The roofs of zinc or lead, con- 
structed of sheets of metal joined together, warp in the 




Fig. 52. — Straightening of the walls at the Conservatoire des Arts et Metiers. 



summer because their expansion is confined by the fasten- 
ings, and tear apart in the winter because their contraction 
is hindered. It is therefore advisable to make one sheet 
overlap another, much in the same manner as tiles, so that 
the metal may freely undergo the changes of dimension to 
which it is subjected. It is equally necessary that gutters 
which are exposed to the air, should not be soldered 
together in too great lengths. Strong stones have been 
broken by the expansion of bars of iron fastened into them. 
The stones even, although their expansibility is not great, 
sometimes separate from each other in very cold weather, 
and again close up when it becomes warm. In bridges a 
lowering or elevation of the arch results from the same 
causes. Suspension bridges are, above all, peculiarly liable 



INTERIOR AND EXTERIOR WORK. I45 

to these dangerous alterations of expansion and contraction. 
A chain of ioo yards long suffers in one year a variation of 
about two inches and a half. These effects must all be 
taken into consideration in the construction of buildings ; 
and. above all, materials expanding unequally must be kept 
separate, because it is the unequal expansion of different 
parts which often est causes the greatest danger to the whole. 

After having thus demonstrated by so many observations 
the power of the molecular forces which exist in solid and 
liquid substances, we conclude that heat, in overcoming the 
resistances of these forces, produces an interior mechanical 
work, and that it is partially spent or consumed in the 
operation, according to the following rule : — 

When a solid or liquid body is heated, and it expands 
whilst overcoming an exterior resistance, such as that of the 
atmosphere, the heat consumed may be divided into three 
parts : the first is expended to produce an exterior work, 
the second to produce an interior work ; the third, passing 
into the body, remains in a state of sensible heat and causes 
an elevation in the temperature. 

If the body which has been heated is afterwards cooled, 
the heat which it disengages comes from the three pre- 
ceding operations, effected inversely : a part is created by 
the exterior resistances when they press on the surface ot 
the body, and compress it little by little. Another part is 
created by the molecular forces, as they draw the molecules 
nearer to each other, and replace them in their primitive 
positions. The remainder of the heat disengaged is the 
sensible heat of the body, which passes outwards at the 
same time that the temperature falls. In the first operation 
an exterior mechanical work is expended, and in the second 
an interior work. 

It is the existence of the interior work which establishes 
an essential difference between gases and solids or liquids. 
Generally, these last-named bodies are not subjected to 
other exterior pressure than that of the atmosphere, and 
then the exterior work may be disregarded : the heat cor- 
responding to the interior work (which is called heat of ex- 
pansion) and the sensible heat, only are considered. 

K 



146 THE PHENOMENA AND LAWS OF HEAT. 



3. How the expansion of bodies is measured. — Maximum 
density of water. 

We shall now proceed to show how the expansion of 
bodies may be measured. To attain this it is necessary to 
invent delicate and extremely sensitive instruments. 

For gases, the most simple instrument has the form of 
Galileo's thermometer (fig. 6, A.) By weighing the amount 
of mercury necessary to fill the bulb, and that which occupies 
one division of the tube, we may calculate by proportion 
how many times the bulb contains the capacity of one 
division. Then, having introduced the gas into the bulb, 
and a little mercury into the tube so as to separate the gas 
from the atmosphere, we shall know how many divisions 
correspond to the volume occupied by the gas. When 
the bulb is placed in water by the side of the thermometer, 
and the water is heated, we may see the little mercurial 
index move, and the number of divisions it passes through 
measures the expansion, whilst the thermometer measures 
the number of degrees that the temperature rises. We may 
thus prove the law of Gay-Lussac : for each degree, the 
volume of any gas whatever increases by ^^ of its original 
volume. 

The same apparatus will serve for liquids. We have only 
to fill the bulb with the liquid, as in the construction of the 
thermometer, and to follow the same rules as with gases. 

In all cases the expansion of the glass must be taken into 
account, as it makes the increase in the volume of the gas 
or liquid in the bulb appear too small. We may convince 
ourselves of this fact by performing the following experiment. 

Take a glass flask, to the neck of which is joined a tube 
divided into equal parts, and fill it with alcohol, coloured 
red, so as to form a kind of enlarged thermometer (fig. 53). 
If we plunge the flask into boiling water, we shall imme- 
diately see the level of the spirit fall ; but later it rises, and 
ascends higher and higher. To explain this : — The glass 
which incloses the liquid becomes hot first; it therefore 
expands, and more room being thus provided, the level oi 



CONTRACTION OF HEATED WATER. 



147 



the spirit sinks. But, gradually, the liquid also becomes 
heated, and, as it expands more than the glass, its level 
rises, not only to, but above its original position. 

It is by paying attention to this effect of the vessel in 
which they were contained, that the coefficient of expansion 
has been measured in several liquids; 
that is to say, the quantity by which the 
unity of volume increases when its tem- 
perature is raised one degree, and it has 
been found that they have not the same 
coefficient. Thus alcohol expands more 
than water, and ether more than alcohol. 

We may remark here that any body 
whatever that has been expanded by 
heat, contracts according to the same 
law in the inverse action of cooling ; that 
is to say, it always resumes the same 
volume in passing through the same 
temperature. 

Water presents a peculiar phenomenon, 
which finds an important application in 
nature. 

Take a large water-thermometer, similar 
to the last-mentioned flask, plunge it into 
water having a temperature of 46 Fahren- Mansion oF Solids.*" 
heit, and let it remain in the water until it 
also is at this temperature ; we shall see the level stop at a 
certain division of the tube, which we shall mark. Now 
immerse a few pieces of ice in the water-bath, so as to cool 
it : the level of the water will descend in the tube as the 
temperature falls ; soon, at about 39 , it will appear stationary, 
but will again rise, and when the bath has fallen to 32 the 
level will have returned to nearly its primitive position. 
Thus the water inclosed in our large thermometer is at its 
smallest possible bulk when at a temperature of 39 . 

We conclude from this that water at 32 Fahrenheit, when 
heated, first contracts, reaching its smallest volume when 
at 39 , and afterwards expands more and more until it 
begins to boil. 




14^ THE PHENOMENA AND LAWS OF HEAT. 

There is, therefore, more water in a quart measure at 
39 Fahrenheit, than at any other temperature ; which we 
express by saying that water attains its maximum density 
at 39 Fahrenheit. It is in consequence of this peculiarity 
that the kilogramme has been fixed on as the weight of a 
litre of water at this particular temperature,* and not at an 
indefinite temperature. 

The celebrated Saussure has noticed that the temperature 
at the bottom of deep lakes is 39 Fahrenheit (4 Centigrade) 
in all seasons ; we now know the reason. During the 
autumn nights, the surface of the water of the lakes is 
cooled ; and as it becomes denser, it sinks, and the lower 
and warmer parts rise to be cooled in their turn. As the 
water is most dense at 39 F. it follows that the parts which 
have fallen to this temperature should sink below all the 
rest, — that is to say, to the bottom. There is, therefore, a 
descending current of cold water and an ascending current 
of warmer water, so that the temperature decreases progres- 
sively from the surface downwards, and is always at 39 F. 
at the bottom. When in the day-time the sun's heat reaches 
the lake, the upper parts in becoming warm become less 
dense, and remain in their place ; further, they absorb the 
solar heat and prevent it from reaching the water at the 
deeper parts of the lake. As colder weather approaches, 
the nocturnal cold becomes predominant, and at last the 
whole bulk of water in the lake will have fallen to 39 F, 
On the arrival of winter the surface-waters become colder than 
39 F., but at the same time becominglessdense, theystill pre- 
serve their position. The temperature increases downwards 
from the surface (where it may be at 32 F., o° C.) until it 
reaches the bottom, where it is always at 39 F. When the 
surface-waters fall to zero (32 F.) it begins to congeal slowly, 
little needles of ice form, and float, because they are less 
dense than the water. Agitated by the wind, they increase 
in size by congealing the water that they touch, and at last 
they join together and form a sheet of ice which covers the 
lake. This glacial covering preserves the lower waters from 

* Namely, 4 Centigrade, or 39 Fahrenheit. In England the standard 
gallon of water weighs 10 lb. at a temperature of 62 Fahrenheit. 



TEMPERATURE OF FREEZING WATER. 1 49 

cooling, and thus their temperature is maintained through- 
out the winter. On the return of warm weather the ice 
melts ; so long as the water produced is at zero, it remains 
at the surface ; it is not until after some time that it receives 
sufficient heat to attain the temperature of 39 F., but there 
is a moment when the water throughout the whole extent 
of the lake will again have this common temperature. 
In the summer the upper parts are warmest, and if the 
lake be sufficiently deep they will prevent the solar heat from 
penetrating to the bottom. The lower parts are again but 
slightly heated by conduction, because water is an extremely 
bad conductor of heat. To sum up, thanks to the maxi- 
mum density of water, heat is, as it were, stored up at the 
bottom of lakes and seas. It there maintains in exist- 
ence an infinity of beings, animal and vegetable, which have 
not therefore to combat either excessive heat or cold. In 
this provision we see an evidence of admirable order, and 
the fact, which at a first glance appeared anomalous, reveals 
itself as the principle of a grand terrestrial law. 

There is no delight more pure than that of the man 
who, after having habituated his mind to reading the book 
of Nature, is allowed to contemplate its beauties. The 
traveller tastes this pleasure when, face to face with a grand 
spectacle which he has sought at the price of a thousand 
fatigues, he reflects on the cause of the marvellous effects 
on which he contemplates, and raises his mind sufficiently 
to discover the law dictated by the Creator. And when he 
has seized this law there are no longer useless details for 
him • he finds in the smallest fact which escapes the atten- 
tion of the listless a verification of the justice of his reason- 
ing. In this spirit, Rumford, traversing the glaciers of the 
Alps, stopped before a natural cavity like a small pit dug in 
the ice. He perceived a stone at the bottom of the water 
which filled this cavity, and in the following manner ex- 
plained how the pit had been sunk. The stone, resting in 
the first place on the surface of the ice, absorbs the heat 
of the sun better than does the ice, and thus melts a small 
portion of the latter which surrounds it. The water arising 
from this fusion then becomes heated to 39° F. (4 C), and 



i5o 



THE PHENOMENA AND LAWS OF HEAT. 



sinks to the bottom of the cavity thus commenced. By 
giving up some of its heat, it there causes a fresh por- 
tion of ice to melt, and thus enlarges the cavity. Having 
then become less dense, it again rises to the surface, is again 
heated to 39 ° F. by the sun, and afterwards sinks to the 



.'■■ul^MlmiNnlmil-nl. 



ii^4 






/ 




Fig. 54. — Apparatus for measuring the Expansion of Solids. 



bottom, and thus continues the formation of the cavity. 
The pit has thus been gradually sunk by this circulation 
of water taking from above the heat of the sun, and carry- 
ing this heat below to the ice. The stone remains at the 
bottom as a silent witness to the circumstances which have 
produced the hole. 

Wq have still to study the methods of rendering the ex- 
pansion of solids measurable. It is so small that special 
instruments are necessary to measure it. Thus, we have 
stated that a decimetre of iron lengthens by about twelve 



UNEQUAL EXPANSIBILITY OF METALS. 151 

hundredths of a millimetre when it is heated from zerc 
(32 F.) to ioo° C. (212 F.): this excessively small expan- 
sion maybe rendered measurable in the following manner: 

An iron bar one metre long is placed in a large reservoir; 
one of its extremities rests against a fixed obstacle, whilst 
the other is brought to bear on the short arm of a vertical 
lever (fig. 54). The long arm of the lever is a thousand 
times longer than the short one, and its motion is measured 
by a horizontal divided rule. The bar of iron is surrounded 
by ice, and the point marked by the indicator on the rule 
is observed. The ice is now taken out, and water put in, 
which is made to boil by heating the bottom of the reservoir. 
The water boils, and the bar of iron is therefore heated to 
ioo° C.,* and consequently the point of the lever advances 
twelve decimetres along the horizontal rule. Evidently, the 
expansion of a bar of iron is a thousand times less ; that is 
to say, the increase in length amounts to twelve-tenths of a 
millimetre. It may be easily discovered by means of this 
apparatus, that the expansion is too times less for one degree; 
the coefficient of the linear expansion of iron amounts to 
To owoo of its length at zero. 

If the bar of iron be replaced by one of another sub- 
stance, a different number will be obtained : each substance 
has its particular coefficient of expansion. 

It is easy to demonstrate the unequal expansibility of two 
metals by making a straight bar with a blade of copper, and 
one of iron, for instance, riveted together (fig. 55). When 
this bar is heated it becomes curved, and the copper is on 
the convex side. It therefore occupies a greater length 
than the iron. When allowed to cool, the bar will again 
become straight. If again it be exposed to very great cold 
it will curve in the reverse direction, and the copper, con- 
tracting more than the iron, will be found to occupy the 
concave side. It has been thought possible to construct 
pyrometers founded on this kind of effect. 

The increase in volume of a solid substance caused by 

heat may be deduced from the increase of length suffered 

by a bar of the substance. Thus, a bar of iron one deci- 

* The boiling point of water is ioo° on the Centigrade thermometer 



152 THE PHENOMENA AND LAWS OF HEAT. 

metre in length, lengthens 3 ooVooo of a decimetre for each 
degree in the elevation of its temperature ; a cubic deci- 
metre would therefore expand for each degree three times 
the amount, — that is to say, tot& 6 otrt of a cubic decimetre, or 



/*" \ 



r — , "1 









; 1 i*?_ir> 




Fig. 55. — Double bar (iron and copper). 

thirty-six cubic millimetres. This result may be demon- 
strated by reasoning, and its exactitude proved by experi- 
ment with the help of peculiar apparatus, which, however, 
need not be described here. 



4. Explanation of various phenomena* 

One very curious effect of the expansion of solids may 
be observed by almost any one. It was discovered in 1805, 
in a foundry in Saxony, by M. Schwartz. A very hot 
ingot of silver had been placed on a cold anvil, and it 
began to tremble, producing at the same time a musical 
sound. This phenomenon was again observed in 1829 by 
Mr Trevclyan, in England, when he happened one day to 
rest a very hot soldering iron on a mass of cold lead. 
Professor Tyndall has repeated the experiment in the fol- 
lowing manner : — 

Two thin plates of lead are fixed in a vice parallel to each 
other; but kept separate by a piece of wood one centimetre in 



VIBRATION OF METALS. 



153 



size (fig. 56). A fire-shovel is then heated and balanced on 
the edge of one of the plates of lead. It will then oscillate 
from one bar to the other, and a sound will be heard which 
may be made very pure if the handle of the shovel be lightly 
supported with the finger. 

We will now explain this phenomenon. 

The lead is warmed at the point of contact with the 
shovel; it expands at this point, and a little elevation is 




Fig. 56. — Trevelyan's Experiment. 



caused suddenly, which makes the shovel see-saw: it falls 
against the second plate, where the same effect is produced; 
the shovel therefore returns to the first, and oscillates so 
long as it remains hot enough to cause a sufficient elevation 
in the piece of lead it touches. This oscillation is a vibra- 
tory motion, which is propagated in the air until it reaches 
the ear, where, if sufficiently rapid, it will produce the 
sensation of sound. The pitch of the sound is higher as 
the oscillations are quicker. 

Mr Gore has made another experiment, which may be 
explained in the same manner. Two copper rails are 
placed at a distance of two centimetres from each other on 
a wooden stand, so that a hollow copper ball may roll easily 
along them (fig. 57). A copper wire is attached to the 
extremity of each rail, the two wires being connected with 



154 THE PHENOMENA AND LAWS OF HEAT. 

the poles of a Voltaic pile. The electric current passes 
through the rails and the ball of copper, and heats the 
rail very strongly at its point of contact with the ball, be- 
cause at this point the resistance to the passage of the 
current is very great. A little elevation is therefore caused, 




Fig. 57. — Gore's Experiment 

and the ball is raised ; it is therefore no longer in equi- 
librium, and at first vibrates a little, but a new elevation 
being formed at each fresh point of contact, it begins to 
roll. 

It may very naturally be asked, if all solid substances 
expand by heat. We have seen water contract when 
heated from zero to 4 C. (39 F.), and it is natural to sup- 
pose that there may also be solid substances which behave 
in a similar manner. In fact, wood and certain argil- 
laceous earths contract by heat : but that is caused by the 
water interposed between their particles, which evaporates 
when heated, and allows the particles to come nearer to- 
gether ; solids of this kind often have pores sufficiently 
large to contain much water. Their contraction by heat 
does not, therefore, resemble that of water at zero. Lately, 
it has been discovered in England that vulcanized caoutchouc 
really contracts by the action of heat on its molecules, 
when it is stretched out strongly and it possesses all its 
elasticity. This property evidently depends on the posi- 
tion of the molecules, which may be such that heat brings 
them nearer together, disarranging them and overcoming 



SPECIFIC HEAT. 1 55 

the interior forces which unite them. The molecules of 
ordinary unstretched caoutchouc are arranged differently 
from those of stretched caoutchouc ; it is not, therefore, 
astonishing that, both being heated, the first should expand 
while the other contracts : this results from the different 
arrangement of their molecules. 

Yet another property of bodies relative to the change in 
their temperature remains to be noticed. We have already 
alluded to it in our study of conductibility, but it is one 
which demands more particular attention. 

5. On specific Heat. 

Take a block of ice, and make two cavities in it. Put in 
one 80 grammes of water at ioo° C, and in the other 
80 grammes of copper, also at ioo° C. You will find, after 
some time, that the ice melted by the hot water weighs 
100 grammes, whilst only a tenth of this quantity is melted 
by the copper. 

Thus copper, in falling from the same temperature as an 
equal weight of water, disengages one-tenth of the amount 
of heat. Reciprocally, we may say that, in being heated 
the same number of degrees as an equal weight of water, it 
will consume one-tenth only of the amount of heat. 

If another similar experiment be made, putting in the 
cavity 80 grammes of water at 50 C. (instead of at 100 ), 
only 50 grammes of ice will be found melted (instead of 
too grammes), so that the quantity of heat disengaged by a 
certain weight of water falling from ioo° C. to zero, is double 
that disengaged by the same weight of water falling from 
50 C. to zero. This quantity of heat is proportional to 
the falling temperature, and it is the same when the opera- 
tion is reversed, and the heat is under consideration which 
corresponds to an elevation of temperature. It results 
from this reasoning, that if we give the name thermal unit 
to the quantity of heat necessary to raise from zero one 
kilogramme of water one degree, two thermal units will be 
necessary to raise it two degrees, 100 thermal units to raise 
it from zero to ioo° C. Now, if instead of one kilogramme 



I56 THE PHENOMENA AND LAWS OF HEAT. 

we take 80 grammes, we shall only require 8 thermal units : 
such is the heat disengaged by the hot water in our first 
experiment. As to the copper, it has only disengaged 
eight-tenths of a thermal unit ; in consequence, to heat one 
kilogramme of copper from zero one degree, a tenth of a 
thermal unit is necessary. It is this quantity which is called 
the capacity for heat, or the specific heat, for example, of 
copper.* 

If instead of copper any other body had been used, we 
should have found its specific heat, as every substance has 
its peculiarity in this respect. Of all solid or liquid bodies 
water requires most heat, taking the same weights, to raise 
it to a given temperature. This is a new property to add to 
those which we have already recognized in this marvellous 
substance. 

Water has already presented itself to us as an immense 
reservoir for solar heat, its duty being to preserve this heat 
and distribute it over the surface of the earth. Now, 
amongst all the substances met with in nature, it is water 
which suffers the smallest decrease in temperature whilst 
disengaging a given quantity of heat. The ocean currents, 
in carrying equatorial heat to cooler regions, cool less than 
would a current of any other liquid, so that it is best 
adapted to give the climate of these regions a milder 
character. For example, with an ocean of mercury the fall 
in temperature corresponding to the same disengagements 
of heat would be thirty-three times greater than with our 
ocean of water. 

Two celebrated French physicists, Dulong and Petit, 
have discovered a remarkable relation between the capacity 
of a body for heat and its chemical constitution. The law 
which they have drawn from this has thrown a new light 
on the intimate structure of matter, and must contribute 
considerably to the acquirement of a knowledge of it. We 
shall terminate this chapter with a statement of the nature 
of this law. 

Chemistry teaches us that an atom of lead weighs about 

* The thermal unit adopted in England is the amount of heat neces* 
sary to raise a pound of water I ° Fahrenheit. 



SPECIFIC HEAT. I57 

as much as three atoms of zinc. Now, the capacity of lead 
for heat is the third of that of zinc. There must neces- 
sarily be a grand law which rules these quantities, when it 
is remembered that other simple bodies present similar 
relations. Suppose that an atom of lead requires as much 
heat as an atom of zinc to acquire a given elevation in tem- 
perature, and we shall have the explanation of the law. In 
fact, three atoms of zinc require three times the above 
quantity of heat, and, as they weigh as much as one atom of 
lead, the zinc will require three times as much heat as the 
lead, taking equal weights. We shall, therefore, conclude 
that the specific heat of the atom of any simple body what- 
ever is a constant quantity. 



CHAPTER VII. 

ON FUSION AND SOLIDIFICATION. 

I. Law concerning the temperature at which substances fuse — 
Heat consumed during fusion and produced by solidification. 

We have shown in our second chapter that if a thermometer 
be plunged into ice which is being melted by the application 
of heat, it will not mark any change of temperature so long 
ts any ice remains ; and, keeping before us the proper 
definition of the word, we say further, that the temperature 
of melting ice is constant. This property, in fact, belongs 
to all other substances which melt by the action of heat. 
To illustrate this, heat some sulphur in a glass flask (fig. 58) 
and immerse the bulb of a mercurial thermometer in it. 
We shall see the level of the mercury rise gradually until it 
indicates no° C. on the scale, when fusion commences. 
After this, the mercury will not rise until the whole of the 
sulphur is completely fused into a liquid mass. The fusing 
point of sulphur is said to be no° C. 

Again, take liquid sulphur at 120 C. of temperature, and 
allow it to cool. At first the level of the mercury in the 
thermometer will fail, but having reached no° C. it will 
stop, and at the same moment solid needles of sulphur will 
appear on the surface of the molten mass and on the sides 
of the vessel which contains it. In fine, the sulphur has 
resumed the solid state throughout, and its temperature has 
remained constant during the solidification. This tem- 
perature is the same as that of fusion. The level of the 



TEMPERATURE OF FUSION. 



*59 



thermometer will not recommence descending until the 
solidification is complete. 

A great number of substances present the same phe- 
nomenon ; only each has its special point of fusion or solidi- 
fication. Thus bees'-wax melts at 
62 C, tin at 235 C, lead at 332 
C, gold at 1,200° C. 

There are infusible substances. 
Some, like carbon, resist the highest 
temperature one can command, and 
are hence called refractory ; others, 
like white lead, marble, and wood, 
are decomposed by the action of 
heat, because their atoms are feebly 
united. The number of the first- 
named class diminishes as the pro- 
gress of science gives us the com- 
mand of higher temperatures ; as 
to the second, we are enabled to 
fuse some of them by submitting 
them to a strong pressure so as to 
prevent the separation of their atoms. 
Marble has thus been melted by 

inclosing it in a gun-barrel her- Fig. 58.— Fusion of Sulphur. 
metically closed by a screw sto pper. 

When the marble is heated under these conditions, it at first 
undergoes a partial decomposition, carbonic acid gas and 
lime being produced. The gas being imprisoned in the gun- 
barrel exerts pressure on the unaltered marble, and main- 
tains its elements in union — this may now fuse. After a 
sufficient lapse of time the apparatus is allowed to cool, and, 
on opening it, appearances of fusion will be found in the 
solid residue. 

In order to solidify a liquid by cold, only one difficulty 
has tc be surmounted : the cold must be sufficiently ener- 
getic. As the means of producing intense cold become 
better understood, we shall be able to solidify many sub- 
stances which at present baffle our skill. Mercury solidifies 
at 40 , and protoxide of nitrogen at ioo° below zero of the 




l6o THE PHENOMENA AND LAWS OF HEAT. 

Centigrade thermometer. We shall see in Chapter IX. that 
these are the substances used for the production of excessive 
artificial cold. Among the liquids which we are still unable 
to solidify, may be mentioned the sulphide of carbon, a sub- 
stance which is much used in the manufacture of caoutchouc. 
The complete fusion of solid substances does not always 
occur suddenly, as in the case of ice and sulphur, which 
become as fluid as possible at the instant of melting. Glass, 
for example, assumes a pasty consistence when its tempera- 
ture is sufficiently high, and it is whilst in this state that it 
can be fashioned by the glass-workers, drawn into thread, 
blown, bent ; in a word, worked into a infinity of shapes. 
Also, alcohol, when subject to excessive cold, becomes 
viscid. The pasty condition does not always appear at or 
about the temperature of fusion • it results from a par- 
ticular arrangement of the molecules, and in some instances 
takes place at other temperatures. This is the case with 
sulphur, which becomes pasty at about 200 C. 

It will be worth our while to give a few moments' atten- 
tion to the leading phenomenon described above ; namely, 
the constancy of the temperature of fusion and solidifica- 
tion, with a view to discover the part played by heat in 
these operations. When a body melts, its molecules are 
evidently separated one from another by the action of heat 
derived from some neighbouring body. As these molecules 
were bound together by interior forces, it was necessary to 
overcome the resistance of those forces; hence, so much 
interior work produced and heat spent. From the fact that 
the temperature remains constant, we should conclude that 
the heat which reaches a substance undergoing fusion does 
not enter it in the state of sensible heat, but is transformed 
as it enters into a mechanical work. This work is interior 
only if there is no outside pressure acting on the surface 
of the body so as to hinder the change in volume which 
always accompanies fusion. If, on the contrary, such a 
pressure exists, — that of the atmosphere, for instance, — the 
exterior work must be taken into consideration ; but usually 
this is so slight compared with the interior work, that the 
latter may be said to exhaust all the heat supplied. The 



HEAT OF FUSION. l6l 

heat thus expended is usually called latent heat; but as such 
an expression, used in former days by the believers in 
caloric, might suggest that this heat really exists, hidden 
somewhere in the body, we prefer to call it by a name which 
simply expresses the observed fact, — namely, heat of fusion. 

In the inverse operation, that of solidification, the con- 
stancy of the temperature is now very easily explained. 
The fluid molecules being subjected to cold, cease to be 
separated from each other by heat so soon as their temper- 
ature falls to the point of fusion ; the interior forces, no 
longer overcome, are again victorious, and reconstitute the 
solid body. In resuming this condition, mechanical work 
is expended, and an equivalent of heat is created. It is 
this heat which, being gradually disengaged, prevents the 
sensible heat of the body from diminishing, and, in conse- 
quence, the temperature from falling. After solidification a 
quantity of heat should be disengaged equal to the heat 
spent during the fusion, the exterior work being neglected 
in our estimate. The following experiments will prove the 
exactitude of this reasoning. 

We already know the heat of fusion of ice : to melt a 
kilogramme of ice at zero would require a kilogramme ot 
water at 79 C. This water is cooled as the ice melts, and 
it falls to zero, after which it ceases to act. At each degree 
of its fall in temperature it disengages one thermal unit ; 
the total heat consumed by the fusion of one kilogramme 
of ice at zero would therefore amount to 79 thermal units. 
The same result would be obtained by replacing the kilo- 
gramme of water at 79 C. by 79 kilogrammes at i° C. of 
temperature. 

A similar experiment may be made with wax, which melts 
at 62 C. Take a large quantity of bees'-wax maintained 
at this temperature, without being allowed to fuse, and 
throw over it 44 kilogrammes of water at 63 C. ; the tem- 
perature of this water will fall to 62 , disengaging 44. 
thermal units, and one kilogramme of wax will be found 
melted. Thus the heat of fusion of wax is 44 thermal 
units, rather more than half that of ice. 

It is equally easy to prove that solidification is accom- 

L 



1 62 THE PHENOMENA AND LAWS OF HEAT. 

panied by the disengagement of heat. Lead melts at 33 2° C. 
Maintain exactly at this temperature one kilogramme of 
the metal in the solid state ; to do this it is simply neces- 
sary to reduce the supply of heat at the instant that the 
metal has attained a temperature of 33 2 C. to no more 
than may be sufficient to prevent the lead from cooling. 
Throw this lead into one kilogramme of water at zero, 
the temperature of the water will rise ten degrees. There- 
fore the kilogramme of lead in cooling has disengaged ten 
thermal units. Make another experiment with one kilo- 
gramme of lead completely melted and at the same tempera- 
ture of 332 C, and throw it into one kilogramme of water 
at zero ; the temperature of the latter will rise fifteen degrees. 
This increase of five degrees indicates a disengagement of 
five thermal units, which did not occur before. It occurs 
because the kilogramme of melted lead first solidifies by 
contact with the cold water before cooling, and disengages 
a quantity of heat equal to that it had absorbed from the 
fire whilst melting. The conclusion from this double experi- 
ment is, that the heat of fusion or solidification of lead is 
five thermal units, sixteen times less than that of ice. 

Every fusible substance has its peculiar heat of fusion, 
and, what is remarkable, ice requires the greatest quantity 
of heat to melt it, as it also has the greatest capacity for 
heat. In accordance with these facts, it comes to pass 
that the frozen soil in winter receives a store of heat, which 
tempers the cooling action of the atmosphere and the celes- 
tial space, and which prevents the temperature from falling 
rapidly below the freezing-point, because each kilogramme 
of water which freezes disengages seventy-nine thermal units. 
When a thaw occurs, the water again takes up this amount ot 
heat, and in so doing tempers the warming action of the 
atmosphere and the sun ; it prevents the temperature from 
rising rapidly above the freezing-point — another cause, in 
addition to those we have before alluded to, of the mildness 
of the climate in regions which abound in water. Cold 
and warmth thus succeed each other less suddenly ; there 
are not, as in countries deprived of water, like Central Asia 
and Australia, excessive cold in winter and excessive heat 



CRYSTALS — ICE-FLOWERS. 163 

in summer. Water, therefore, may be called the natural 
regulator of temperature on the surface of the earth. 

The slow melting of ice is a consequence of the fact that 
this substance requires a great amount of heat to melt it. 
Thus a layer of ice on the surface of a body is often very 
efficacious as a protection against heat : the temperature of 
the body cannot rise above the freezing-point until the 
whole of the ice is melted. Inversely, a layer of water acts 
as a preservative against cold in consequence of the slow- 
ness of its congelation. Envelope a body in wet rags, and 
keep the latter soaked with water, you will prevent the body 
from cooling below the freezing-point, even if exposed to 
a very sharp cold \ the water will form little icicles slowly on 
the linen, whilst continually disengaging heat. This method 
is available for the preservation of organic substances from 
frost during winter. 

Some curious examples are on record, illustrating the 
slowness with which ice melts. In the winter of 1740 a 
palace was constructed in St Petersburg of ice taken from 
the Neva in large blocks. As gay parties were assembled in 
this palace, evidently a large quantity of heat must have 
been accumulated within, which slowly melted the superficies 
of the walls ; but the fusion was very slow, and the walls 
being sufficiently thick, lasted for a long time. Cannon, 
with barrels four inches thick, were also made of ice, from 
which iron bullets were fired, yet the cannon were not 
melted nor broken by the explosion of the powder. In 
Siberia sheets of ice are used for windows ; their interior 
surface does not melt, because the outside is so excessively 
cold as to keep the temperature of the whole below freezing- 
point. 

What we mean by heat of fusion is sufficiently exemplified 
by the facts just stated. We have now to seek experimental 
proof of the interior work effected in the operations of 
fusion and solidification. 

2. Liter lor Work. — Crystals. — Ice- Flowers. 

Melt some sulphur in a large earthen crucible, and allow 
it to cool without agitation. It will cool very slowly, and 



164 



THE PHENOMENA AND LAWS OF HEAT. 




the temperature will fall to no° C, first on the surface and 
on the sides of the crucible, the central part still remaining 
liquid after the solidification has commenced. Little needles 
of sulphur will be seen to form on the surface, crossing each 
other in all directions. At this moment, break through the 
centre of the crust thus being formed, and turn the crucible 

over ; the liquid sulphur will run 
out, and the solidified part be found 
adhering to the sides in the shape of 
delicate yellow transparent needles, 
all pointing towards the centre of 
the crucible (fig. 59). These are 
called crystals of sulphur, and by 
carefully studying their form it has 
been discovered that each crystal 
is terminated by plane faces regu- 
larly disposed. 

A law therefore presides over the 
arrangement of the molecules ; when 
solidification takes place there are 
motive forces which cause each 
molecule to take a determinate posi- 
tion. But the movement of a molecule under the influence 
of a force is a mechanical work expended ; and the whole 
of these molecular movements constitute what we call in- 
terior work. We have, therefore, proved the existence of this 
work by arresting it whilst under execution, and by showing 
the interior state of the body at that moment. If we had 
allowed the sulphur to cool entirely, this work would have 
been continued silently, hidden by the superficial crust, and 
the admirable structure presented by the interior would have 
escaped our notice. The molecular forces are consummate 
architects, always working in obedience to laws dictated by 
the Creator. 

It is not always easy to crystallize bodies by fusion, and 
our want of success must be attributed to the imperfection 
of our processes. Most often the crystals are very small, 
and so grown into each other that the cleavage presents no 
indication of crystallization : this happens when the liquid is 



Fig. 59. — Crystallization 
of Sulphur. 



ICE-FLOWERS. 1 65 

agitated whilst cooling. When the crystals are sufficiently 
large the cleavage opens to view their brilliant facets, and 
people have no hesitation in calling that body crystalline. 

Here again it is water which offers us our best example, 
as if the Creator had designed this wonderful substance to 
contain in itself, and reveal to observers, the most hidden 
natural mysteries ! When the temperature of the air is suf- 
ficiently low, the water it contains is condensed into small 
regular crystals, which sometimes form a fine white dust, so 
perfectly transparent at times as to be invisible ; it is known 
to exist, however, by the sensation it creates striking against 
the face. This condition of ice has been noticed by MM. 
Barral and Bixio in a balloon ascent. Transported into a 
moister atmosphere, these little crystals condense water on 
their surface, and become larger, still preserving their regu- 
larity ; little by little they grow into flakes of snow of the 
most various shapes. The celebrated astronomer Kepler 
was the first to study these forms with care ; navigators are 
especially enabled to study them completely in the polar 
regions, where the snow falls frequently and at different 
temperatures. Scoresby has drawn 96 forms of snow, and 
now several hundreds are known, which are classified. All 
present the aspect of stars with six rays, symmetrically 
modified with relation to the centre of the star (fig. 60). 

The freezing of water at the surface of the earth fol- 
lows the same law ; but the crystals are so joined one to 
another as to form masses of ice, in which it is usually 
impossible to discern them. 

The following is a beautiful experiment made by Mr 
Tyndall, whom we shall assist in a kind of dissection of 
compact ice, and who will make visible to us the interior 
work executed during fusion. We have seen how the solid 
edifice was constructed, we shall now see it as regularly 
destroyed ; its various parts being separated in the same 
order of succession as that in which they were assembled 
together, but inversely. 

A transparent piece of ice is taken, having two natural 
parallel faces, which are those which were horizontal whilst 
the piece of ice was being formed on the surface of the 



166 



THE PHENOMENA AND LAWS OF HEAT. 



water ; they are called planes of congelation. A pencil ol 
solar rays is directed horizontally on this ice with a mirror, 
the ice being placed so that its planes of congelation may 



HP 



Fig. 60. — Various forms of Snow. 




FlG. 6s* — Dis section of Ice by the Solar Rays. 

be parallel with the rays, or, in other words, that the rays may 
enter by the cleavage. Afterwards place a convergent lens 
in the path of the rays in such a place that the focus may 
be formed in the middle of the piece of ice, and notice 
what passes in the ice with a strong magnifyingglass (fig. 6i> 



*» k J ** Jfe* 



*5JT! ... V. «>)_ «i 




Tk 







t 



|4 



*p 






£y^ 



e^4x 






V 



..-*?/ 



& IF' 



I 









% 



JT& 



VK J&l ^%, 



M 



Fjg. 62.— IcoFloweitk 



DISSECTION OF ICE. 1 69 

Where the rays enter, little stars with six branches, some- 
what like flowers, appear rather closely grouped ; follow 
them with the eye to a little distance within the surface at 
which the rays enter, they will be found more distinctly 
grouped, which distinctness becomes greater at a still more 
interior point. These stars may be seen to form and 
gradually develop themselves. First a little brilliant point 
appears, which becomes the centre of a round spot ; after- 
wards the rays appear : these increase little by little, and 
their edges take the form of fern-leaves (fig. 62). The ex- 
planation of this phenomenon is as follows : — 

The solar rays carry heat to the middle of the ice ; but as 
it gradually penetrates, all the calorific rays capable of being 
absorbed by the ice are removed, and at a certain depth 
they cease to have heating powers. This property of 
radiant heat we have already studied in our fourth chapter. 
The ice then is more heated near the point where the rays 
enter than at a certain distance from this point. 

Each brilliant point that appears is due to the commence- 
ment of fusion at that spot. As the fusion continues round 
this point the star appears. Each row of molecules is 
detached in its turn, and as they were arranged symmetri- 
cally in three principal directions, we find in the rays oi 
the liquid star the indication of this disposition of the 
molecules. 

It is in these three directions that the molecular forces give 
way soonest to the action of heat, as if they were those in 
which the struggle was concentrated. We are able to dis- 
tinguish the water produced by fusion, because it reflects 
light towards our eye, and we perceive in the centre of the 
star a point having a metallic appearance, because it is 
formed by an empty space, which reflects light abundantly. 

Why does this empty space occur ? We already know that 
ice is less dense than water ; each star consists of a small 
bulk of ice which has been melted, and the water produced 
occupies a smaller bulk. It is thus that we find in the 
details of this phenomenon much useful instruction. Nature 
is one grand harmony. In the words of Mr Tyndall, 
11 she lays her beans in music, and it is the function of 



IJO 



THE PHENOMENA AND LAWS OF HEAT. 




Fig. 0$ — Exhibition of Ice- Flowers 
by projection. 



science to purify our organs so 
as to enable us to bear the 
strain." * 

The formation of ice-flowers 
may be made visible to a great 
number of persons at once. 
Solar rays must, in this case, 
be made to enter the piece 
of ice perpendicularly to the 
planes of congelation, and a 
lens be also placed on the 
other side of the ice (fig. 63), 
so that the reversed image of 
the ice may be projected on a 
white screen. The experiment 
is to be made in the dark, and 
after some wavering, the stars 
will be found represented on 
the screen ; they are detached 
as faint shadows on a light 
ground, because the water ab- 
sorbs more light than the ice; 
the empty space is marked by 
a white point, because it ab- 
sorbs no light whatever. The 
solar rays may be substituted 
by those of the Voltaic arc, or 
of a Drummond's lime-light. 

3. Chaiige in volume and exte- 
rior resistance. — Expansive 
force of ice. 

The change in volume 
which accompanies fusion re- 
sults from the new arrange- 
ment of the molecules, which 
takes place when they are 



* Heat as a Mode of Motion, p. 112, 



CHANGE IN VOLUME DV FUSION. 171 

under the action of heat, and the character of this change 
depends on their form. There is no a priori reason for 
supposing that a body should augment in volume by fusion. 
Experience decides this question, and reasoning may after- 
wards lead us to some conclusion respecting the form of 
the molecules. 

It will be sufficient to remark that ice floats on the water 
provided by its own fusion, to convince us that it diminishes 
in volume when melting : because a body which floats on 
the surface of a liquid must have a density inferior to that 
of the liquid, and, in consequence, a certain weight of this 
liquid occupies a smaller volume than the same weight of 
the body. For example, it has been found that a kilogramme 
of water at the o° C. (3 2° Fahrenheit) occupies about a thou- 
sand cubic centimetres, and that the same weight of ice at 
the same temperature measures 75 cubic centimetres more. 

The fusion of sulphur offers an example of the contrary 
effect. The solid fragments of sulphur not yet melted, sink 
to the bottom of the sulphur that has been already fused. 
It follows that they are denser than the liquid, and hence 
that sulphur expands when it melts. 

The behaviour of some other fusible bodies resembles 
that of ice : for example, bismuth and type metal ; some, 
like sulphur, expand — and the number of the last is very 
great. Inversely, the liquids of the first class expand on 
solidifying, and those of the second contract. Metals of 
the first class are best adapted for castings, because at the 
moment of solidificaton the expanding liquid exactly fills 
all the cavities of the mould, and in consequence faithfully 
reproduces its details. 

It is the character, in the change of volume to greater or 
less which determines that of the exterior work when the 
body is subjected to a surface pressure. We have neglected 
to take account of this work in the case of substances fusing 
in the atmosphere. But bodies may be submitted, with the 
help of convenient apparatus, to very great pressure ; the 
overcoming of these pressures influences, to a notable ex- 
tent, the heat of fusion. 

In the case of ice, there is, at the time of contraction, an 



172 THE PHENOMENA AND LAWS OF HEAT. 

expenditure of exterior work, and a creation of heat. This 
heat is employed to fuse a part of the body ; consequently, 
the heat coming from without serves to melt the rest, and 
less is required than if the ice were free from pressure. 
Compressed ice may be said to melt more easily than ice 
under ordinary conditions, and it may be supposed that the 
temperature of fusion is no longer the same as that at which 
it froze. In fact, M. Mousson has seen it melt at 1 8° below 
zero Centigrade (about o° Fahrenheit) under a pressure of 
several thousand atmospheres. 

In the case of wax, on the contrary, which ordinarily 
melts at 63 C, and expands by fusion, the exterior work is 
produced by the expansive force of the molecules of the 
body, which overcome the exterior pressure ; this work 
consumes heat. In consequence, the heat coming from 
without serves partly for the execution of this work, whilst 
the remainder is consumed in that of the interior work. 
Compressed wax may be said to melt less freely than 
ordinary wax, and it might be supposed after our expe- 
rience in the case of ice, that the temperature at which wax 
melts when under pressure is above 63 C. This has been 
found to be the case by Bunsen. 

That which happens in the case of the solidification of 
liquids relative to the exterior work, should now be found 
easy of comprehension. It is only necessary to imagine the 
reverse of the above effects. 

Such is the relation found to exist between the two kinds 
of mechanical work effected during the passage from the 
solid state to the liquid state, and vice versa. A number of 
phenomena will now be explained which otherwise would 
appear exceptional. The discovery of this relation is an 
immense step in the study of the intimate constitution ol 
bodies. 

We meet with a fresh proof of the energy of the interior 
forces in the expansive power possessed by the solid or 
liquid bodies, at the moment in which they pass from the 
one state to the other. Try to melt sulphur in a vessel 
hermetically closed, and made of some resisting material, 
we shall find that the vessel will be broken, supposing it to 



EXPANSIVE FORCE OF ICE. 



173 



have been filled. The molecules of sulphur find themselves 
opposed to two contrary forces : the heat arriving from 
without, which tends to separate them from one another, 
and the resistance of the vessel which opposes itself to this 
separation, — and the last is conquered. 

Some very curious experiments of this nature have beer 
made with water, which expands on solidifying. Fill a cast- 
iron tube with water, closing it strongly with a screw stopper, 
and expose it to considerable cold. As the water loses its 
heat, its molecules change their position, and tend to con- 
stitute ice ; but for this they require greater space than that 




Fig. 64. — Expansive force of Ice. 



afforded by the tube : they therefore press against its sides, 
so forcibly as to split it, through its whole length, with 
somewhat of an explosion. 

An officer of artillery made the following experiment al 
Quebec. Having filled a bombshell, about fourteen inche » 
in diameter, with water, he closed it by driving an iron peg 
firmly in, and left it exposed to frost. The stopper whs 
soon driven out to a distance of more than a hundred yards, 
and a cylinder of ice, eight or nine inches long, issued at 



'74 



THE PHENOMENA AND LAWS OF HEAT. 



the opening. On another occasion the stopper resisted, the 
bomb was rent circularly, and a ring of ice was forced 
through the crack (fig. 64). 

This example shows us how formidable may be the effects 
of frost. In winter, our jugs and water-pipes being filled 
with water are often broken ; the earth saturated with 
moisture at the moment of frost swells up and causes 
houses to fall ; porous stones break to pieces upon the 
freezing of the water they contain • trees split up with an 
explosive sound in times of very great cold if their vessels 
happen to be filled with sap. The freezing of plants is a 
disorganization in the consideration ot 
which attention should be paid to this 
effect, although experiments have been 
made with certain aquatic plants which 
would lead us to conclude that this 
effect is not a necessary cause of their 
destruction. 

Water freezing in an open vessel 
may break the bottom. The fact is 
explained as follows. During a severe 
frost ice is formed on the surface of the 
water, and acts like a stopper • the 
water situated below will thus be con- 
fined in a limited space, and when it solidifies, in its turn, 
it presses against the vessel and the covering of ice. If 
the latter is so thick as to excel in resisting power, the 
bottom of the vessel must give way (fig. 65). 

This observation recalls an experiment which demon- 
strates that melted wax contracts in solidifying. Some 
melted wax is poured on to a surface of water contained 
in a cylindrical vessel. The wax being less dense than the 
water, remains on its surface, solidifies on cooling, and 
forms a crust smaller than the vessel ; this crust does not 
adhere to the sides, and falls out loosely on turning the 
vessel over. 

When a shallow surface of water freezes throughout its 
entire thickness on ground presenting little inequalities, 
eminences may be observed to rise above the deeper parts 




Fig. 65. — Freezing of 
Water in an open vessel. 



REGELATION OF ICE. 1/5 

(fig. 66). This happens because the expansion is so much the 
greater as the volume of water under consideration is itself 
greater, and, moreover, because water in solidifying expands 
freely in a vertical direction; it is, therefore, in this direction 




Fig. 66. — Freezing of Water above an uneven surface. 

that one might expect to find any unequal augmentation 
of thickness proportional to the depth of the part. 

We might multiply examples, but those we have cited 
are sufhcient to show and explain the varied effects result- 
ing from the change in volume which accompanies fusion 
and solidification. We shall pass on to the study of some 
other properties possessed by ice, which find very remark- 
able applications. 



4. Regelation. — Glaciers. 

On pressing two pieces of ice together they soon become 
firmly joined ; a third piece may in the same way be joined 
to these, and thus any number of small pieces may be 
formed into one continuous piece of ice. Each piece of 
ice being in a melting state at its surface, regelation takes 
place at the point where two pieces join. Faraday drew 
attention to this phenomenon, and Professor Tyndall has 
studied it completely in later years. We owe to him the fol- 
lowing experiment somewhat analogous to the preceding. 

A box-wood mould being filled with small pieces of ice, 
is submitted to a strong pressure (fig. 67). A perfectly con- 



176 THE PHENOMENA AND LAWS OF HEAT. 

tinuous and transparent block of ice is thus obtained, 
having the form of the cavity of the mould. By using suit- 
able moulds, lenses, spheres, cups, or statuettes of ice may 
thus be made. 

It is natural to seek the explanation of this last experi- 
ment on the effects of compression. We have learnt that 




Fig. 67. — Moulding Ice. 

ice when compressed melts at a temperature below its 
freezing-point ; in consequence, supposing two fragments of 
ice at this temperature submitted to pressure in the mould, 
we may imagine that at the point of contact fusion takes 
place, because the temperature of the ice is above that a* 
which it melts when under pressure. The water arising 
from the fusion is therefore below the usual freezing-point ; 
it fills up the interstices of the fragments, and being thus 
freed from the compressing force — because the fragments 
do not transmit the pressure equally in every direction — it 
again becomes solid. Continuity is thus gradually estab- 
lished throughout the mass: The two facts upon which 
this theory is founded are true : water at the freezing-point, 
or even a little below, when under strong pressure cannot 
be in the solid condition ; water below the freezing-point, 
not under pressure, cannot keep the liquid state. But if 
these laws take a part in the regelation effected within the 



REGELATION EXPLAINED. 1 77 

mould, they can scarcely be supposed to influence the first 
experiment where pressure was exerted by the hands, simply 
to insure contact; this slight pressure would surely be in- 
sufficient to lower the temperature of fusion to any con- 
siderable extent. 

The following explanation of one of the principal causes 
of regelation by simple contact is given by Tyndall. He 
took a piece of ice containing natural cells in which a liquid 
part could be distinguished, and also a gaseous part, which 
always rose to the upper part of the cell (fig. 68 a). By 
plunging the piece of ice in warm water and attentively 
watching its fusion, Tyndall saw the cells diminish con- 
siderably in size at the moment in which their envelope of 
ice melted, when each let a little bubble of air escape, 
which, excessively reduced in size, rose to the surface of 
the warm water. It must be concluded from this that the 
liquid in the cell arises from the fusion of the ice originally 
surrounding the bubble of air ; by its contraction it has 
enlarged the capacity of the cell in which the bubble of air 
remains in a rarefied state. But now how can fusion be 
determined around a bubble of air without the surrounding 
parts ceasing to be solid ? 

To answer this question, Tyndall exposed a piece of ice 
similar to the preceding to considerable cold. The liquid 
part of the cells congealed, and the bubbles of air dimin- 
ished in bulk, thus confirming the preceding theory. He 
next placed a piece of ice in a warm but dark room. After 
some hours the bleb of water made its appearance in the 
cells. Heat can therefore pass by conduction from without 
to the interior of a piece of ice, and thus melt the sides of 
the cells. 

Lastly, by exposing to the rays of heat from a fire a piece 
of ice containing air-bubbles, Tyndall observed fusion to 
take place very rapidly around each bubble of air, and the 
side of the cells to take indented forms, sometimes very 
pretty, in the superficial parts first reached by the radiant 
heat (fig. 68 b). He could not obtain the same effect w T ith 
the obscure rays of heat. Thus fusion may also be deter- 
mined in the interior of ice by the radiation of luminous heat. 

M 



i 7 8 



THE PHENOMENA AND LAWS OF HEAT. 



We conclude from these experiments that heat can 
penetrate a block of ice, either by conduction or radiation, 
and determine fusion by coming in contact with the bubbles 
of air imprisoned within the mass, whilst leaving the sur- 
rounding ice in the solid state. This may be explained as 
follows : the molecules situated in the midst of other mole- 
cules of ice, are less free in their movements than those 





Fig. 68. — Air-celis in Ice. 



which are in contact with the air. When heat reaches the 
exterior surface of a block of ice, a part acts on this sur- 
face, and another part continues to travel till it reaches 
the bubbles of air ; there only does it produce an effect, 
exactly as a blow delivered at one end of a row of billiard 
balls causes the last ball at the other end to jump forward, 
whilst the intermediate balls remain at rest ; each of them 
receives the impulse, and transmits it to the next without 
itself moving ; the last only is put in motion, because it does 
not encounter an obstacle. The fact observed by Tyndall 
may be expressed in a general manner in the following 
words. The fusion of a solid body is easier in the super- 
ficial parts, and in those which present a solution of conti- 
nuity, than in those which are altogether continuous, as if 
the temperature of fusion was. less high on the surface of a 
solid than in its interior. 

To apply this remark to the phenomena of regelation. 
We place two pieces of ice in contact : the two surfaces 
being kept together exercise one over the other a coercive 
action which opposes itself to the continuance of fusion. 
The surface molecules lose their liberty by ceasing to be 



FORMATION OF GLACIERS. 1 79 

superficial, and becoming interior ; their mutual attraction 
re-establishes the solid condition, and they become cemented 
together, so long as sufficient heat does not reach them 
to raise their temperature above the freezing-point. The 
two pieces are thus united into a single block, and fusion 
will not take place at any point of the homogeneous ice, 
until the neighbouring parts are melted in regular order 
from the superficies. 

The child who makes a snow-ball repeats the experiment 
of regelation. The flakes of snow become converted into 
little pieces of ice which unite one to another ; the hand 
crushes them, and changes their position ; they regelate 
afresh, and in this manner the light and delicate snow 
becomes converted into a hard and compact body. 

The traveller who visits the glaciers of the Alps, en- 
counters a deep crevasse : he collects snow at the edge of 
the precipice, he makes a bridge of it, then he mounts this 
improvised structure, and slowly advances over the abyss. 
The frozen snow bends under his weight ; its parts sepa- 
rate and again regelate ; the compressed mass becomes 
firm, and the passage may be effected in comparative safety. 
Let us here pause for a moment, to regard the magnificent 
spectacle presented by the eternal snows ; it is not merely 
a spectacle, but a phenomenon which presents us with a 
wonderful example of the properties of ice. On the tops of 
high mountains the aqueous vapour in the atmosphere con- 
denses in the form of snow, which eternally covers their crests. 
Sometimes the masses of snow descend the declivities as 
avalanches, with a noise as of thunder, and fill the valleys ; 
sometimes they slip along slowly, and accumulate at the 
base of the declivities in compressed masses. The air 
imprisoned in the flakes of snow is little by little expelled, 
and the mass becomes still more solid ; it presses heavily 
on the rocks at the bottom of the higher valleys, and 
descends gradually towards the lower. Here, in conse- 
quence of the increased warmth, it again attains the tem- 
perature of zero, and commences to melt. Then regelation 
takes place on an immense scale, because the bottom of the 
valley and the sides of the mountain present an obstacle to 



l8o THE PHENOMENA AND LAWS OF HEAT. 

the gliding of the ice. Continually impelled by weight, its 
mass may break in overcoming the obstacles to its progress, 
and immense and deep transverse crevasses result from the 
rupture. Soon, however, the downward pressure of the 
consolidated snow and ice causes the sides of the crevasse 
to meet, and regelation takes place. In the meantime other 
crevasses are produced by the same causes, and, the down- 
ward pressure continuing, are again compactly closed by the 
law of regelation. Thus the glacier moves slowly down the 
valley at a rate of progress varying from fifteen inches 
daily in the winter, to thirty inches in the summer, 
dragging here and there the debris of rocks which have 
yielded to its efforts. Having thus descended low enough 
to a warmer region, it melts at its surface and in its depths, 
and becomes the source of a river. A movement, as eternal 
as the snow, continually brings down fresh masses of ice, 
which gradually melt, whilst higher up the loss by fusion is 
compensated by frequent snow-falls. From this description 
of the facts, it is obvious that the glacier, properly so-called, 
is below the line of perpetual snow ; whilst above that line 
is the mass which feeds it, called the neve. 

For a long time it was supposed that glacier ice was of 
a viscous nature, or plastic, like clay mixed with water : this 
was the best explanation that could be offered of the 
phenomena it exhibited in its gradual descent, moulding 
itself to some extent to the shape of the valley which it 
filled. But the experiments of which we have just spoken, 
give the true explanation. The ice breaks into fragments 
as it descends, and these fragments regelate at the points 
where they again come in contact 



CHAPTER VIII. 

CONCERNING EVAPORATION AND EBULLITION. 

I. Superficial evaporation of solids and liquids. 

Certain solids and liquids are volatile, vapours rise from 
their surface which spread into the surrounding space, and 
which have the general properties of all gases, expansibility 
and compressibility. By virtue of the first their particles 
always tend to separate one from another, and by virtue of 
the second they are very easily compressed, and made to 
occupy a less space. The phenomena which constitute 
evaporation can sometimes only be recognised by means of 
experiment. We can only observe it in ordinary circum- 
stances, when the vapour has some characteristic action on 
our senses. To return to an example already indicated in 
Chapter I. : a piece of camphor is inclosed in a well-corked 
flask, or bottle ; its volatility is hardly to be recognised by 
the first glance at it. But uncork the flask, the characteristic 
odour of this substance is immediately manifested, and you 
conclude from this that some of its particles have been 
detached, and have produced the sensation of smell by 
coming in contact with your nose. Giving a little more 
attention, the flask, if it be not too small a one, and if the 
piece of camphor be allowed to remain a short time, a slight 
deposit of little brilliant particles may be observed on some 
parts of its sides. On presenting these parts to the fire, the 
deposit will disappear, and will be re-formed on the cool 
side of the flask; 'that is to say, on the side furthest removed 
from the fire. If the heat still continues to act, and reaches 



l82 THE PHENOMENA AND LAWS OF HEAT, 

the piece of camphor at the bottom of the flask, the deposit 
will augment on the cool side ; the particles will grow into 
little crystals of a geometrical form, and at the same time 
die piece of camphor at the bottom will have diminished 
in volume. With a sufficiently correct balance, it will be 
easy to prove that this piece has lost some of its weight, 
and that the loss is equivalent to the weight of crystals 
deposited on the sides of the vessel. You will thus have 
made a true physical experiment. Having become aware 
of one of the properties of camphor by simple observation, 
you have studied this property by modifying the circum- 
stances in which the phenomenon has occurred, and you 
have thus discovered one of its laws. It remains for you 
to complete your study by a theory which will account for 
the fact. The particles of camphor may be said to be 
transported, by the influence of heat, from the surface of 
the solid piece, to the side of the flask. They were first 
detached from this surface, separated from one another, 
thrown out in some manner in every direction ; they then 
collected together at a part where there was not sufficient 
heat to prevent their union, and reproduced solid camphor. 
We cannot see the particles during their transport, on 
account of their extreme minuteness, and their diffusion, 
but we have proved the final result of the transport, and 
we conclude that heat volatilizes camphor, causing it to 
assume the condition of gas or vapour, invisible like the 
air ; while cold reduces the vapour of camphor to the solid 
state. We observe two inverse transformations, analogous 
to the fusion and solidification of liquids. 

By repeating the same kind of experiments on a number 
of solids, the same conclusion would be arrived at. Iodine 
substantiates the preceding theory, by the production of 
a beautiful violet-coloured vapour. This brownish sub- 
stance, when slightly heated in a flask, fills it with violet 
vapour, which slowly condenses in the form of little brown 
crystals on the cooler portions of the surface. Remove 
the heat altogether, and in a short time the vapours will 
disappear, leaving the unvolatilized portion of the iodine 
at the bottom, and the crystalline deposit on the sides. 



EXPERIMENTS IN VAPORISATION. 1 83 

But the most numerous examples of vaporisation are to 
be met with in liquids. We have already mentioned several 
instances in our first chapter, and we shall now pay studious 
attention to this phenomenon. 

Heat a small vessel filled with water ; we shall soon 
observe a slight cloudy vapour above its surface. This 
cloud is not really vapour : it is composed of a great num- 
ber of little drops of water, which may be collected, for 
instance, on a pane of glass, where they form a liquid layer. 
The phenomenon is analogous to that we observed in the 
case of camphor and iodine. Under the influence of heat 
the surface-particles of water are separated from one another; 
they rise from every part in the form of invisible gas, and 
disseminate into the air. But coming in contact at a cer- 
tain height with colder air, they condense again, because 
heat does not then oppose their union, and they form the 
light cloud which we see, and which really consists of minute 
water-drops. We may conclude from these observations 
that heat causes water to evaporate, and, inversely, that 
cold condenses the vapour thus formed. 

Is it necessary in order to produce vapour, or separate 
the superficial molecules of a body, to apply heat ? Do the 
molecules yield only to heat, to the natural antagonist of the 
force which binds them together ] The answer to these ques- 
tions will be found in our further study of the phenomena. 

The little cloud above our hot water seems to be per- 
manent. If the atmosphere be still, it is but slightly agitated, 
it breaks up, disappears at one point and reappears at 
another ; it resembles a light, mobile body floating in the 
air. On looking at it a little more closely, we find that the 
cloud is not always the same, but that it rises and dis- 
appears, and thus gives place to a new cloud, which is seen 
to rise in a form different from the first. It is the continued 
renewal of the drops of water at nearly the same spot that 
gives the appearance of a persistent cloud ; looking at it 
from a distance, we were deceived by an optical illusion, 
and we now see what really happens of which the following 
is the explanation : 

When a tinv drop of water has been formed by the cooline; 



184 THE PHENOMENA AND LAWS OF HEAT. 

of the vapour, it rises a little higher, being drawn up by 
an ascending current, due to the heating of the air next 
the surface of the hot water, and by the mixing of some of 
the vapour with this air, — two circumstances which diminish 
their density. Reaching the higher and drier air, the drop 
thus raised evaporates, and resolves into invisible gas, which 
mingles with the air ; it therefore disappears. The less dry 
the air of the room is, the higher will the drops rise before 
evaporating, and the thicker will the little cloud appear to 
be. If the air were excessively humid and very calm, the 
cloud might form a column of drops of water, continuously 
produced at the bottom, and rising to the ceiling of the 
room, where they would condense. 

From this we should conclude that water evaporates 
spontaneously in dry air, without the co-operation of heat, 
and we are led to perform some experiments which confirm 
our theory. These experiments are, moreover, indispen- 
sable to enable us to generalize our conclusion by extending 
it to all volatile liquids. The observations we have just 
made on water would not be possible with all liquids, be- 
cause the drops which result from the condensation of a 
vapour are not always visible ; to make them so, the con- 
currence of complex circumstances is necessary, and even 
the vapour of water does not always condense in the form 
of fog, as has already been seen in our fourth chapter, 
where we have spoken of the evening damp. 

2. Elastic force of vapours, — Papitis Digester. 

Expansibility and compressibility are the characteristic 
properties of a gas. From this we conclude that a volatile 
substance, introduced into a vessel quite empty of air or other 
matter, would immediately fill the vessel ; that is to say, that 
the superficial molecules would separate without hindrance 
by virtue of their natural expansive force, and would strike 
out against the sides : they would be then stopped, and, not 
being able to separate to a greater extent, they would press 
against the sides. This is the simplest method of evapora- 
tion, and must be perfectly realized to the mind. 



EXPERIMENTS IN VAPORISATION. 



185 



To illustrate this, take a glass vessel furnished with a stop- 
cock and a glass tube bent into two vertical branches (fig. 
$9). Some mercury is put in this tube, and, having fitted 
a communicating tube between the stop-cock and an air- 
pump, the air contained in the vessel is removed. The level 
of the mercury in the open 
branch of the tube will then 
be seen to be about thirty 
inches below the other level. 
It is this column of mercury 
which balances the atmos- 
pheric pressure, as in the 
barometer. This done, a fun- 
nel is adjusted to the stop- 
cock and filled with a liquid, 
— ether, for example. On care- 
fully opening the stop-cock, a 
small quantity of the liquid is 
allowed to enter when it must 
be immediately closed. If only 
a very small quantity of ether 
is introduced, no trace of liquid 
will be seen in the reservoir : it 
will have been instantaneously 
reduced to vapour, and the 
summit of the column of mer 
cury will establish itself, for 
example, at twenty-four inches 
higher than the lower level. 

The mercury has evidently been driven forward by the 
ether vapour, the elastic force of which is thus manifested. 
Further, the measure of this elastic force is the diminution 
which the column of mercury has suffered, — that is to say, 
about five inches. 

If a further quantity of ether be introduced into the vessel, 
the temperature being at 6o° R, the height of the column 
of mercury will be seen to diminish still further, the con- 
clusion from which must be that the elastic force of the 
vapour has augmented. No trace of liquid will remain so 




Fig. 69— -Apparatus for tie 
vaporisation of liquids. 



1 86 THE PHENOMENA AND LAWS OF HEAT. 

long as the column of mercury stands above sixteen inches 
in height. But so soon as it is at this height, any additional 
liquid introduced will fall into the reservoir without eva- 
porating. A certain quantity of ether is therefore, by its 
complete evaporation, capable of filling the vessel, and the 
elastic force of the vapour is as great as possible when it is 
in the presence of an excess of its liquid. In the above 
example this elastic force is equivalent to the pressure of a 
column of mercury of about fourteen inches in height, since 
in the bent tube the height of the mercury has fallen from 
thirty inches to about sixteen. 

It is easy to conceive how the reservoir becomes saturated 
with a limited quantity of vapour, by supposing that the 
superficial molecules of a volatile liquid tend to separate 
from one another with a certain force which is itself limited. 
When the vapour formed attains sufficient elastic force, it 
balances the force of separation of the molecules situated 
on the surface of the remaining liquid ; these then remain 
in the liquid state and evaporation ceases. 

The above experiment may be made with any volatile 
liquid, and the results will only differ in the height of the 
column of mercury which measures the elastic force of their 
vapours. 

The quantity of vapour which a given vessel is able to 
contain is evidently proportioned to the capacity of the said 
reservoir ; a vessel measuring two quarts would hold twice 
as much vapour as a vessel measuring one quart. When an 
indefinite quantity of liquid is introduced into an empty 
vessel, one of three things may happen : the quantity in- 
troduced may be exactly sufficient to saturate the vessel, 
and then all the liquid will be reduced into vapour; or 
it may be less than sufficient, in which case the vessel will 
not be saturated ; or lastly, it may be more than enough, 
when an excess of liquid will remain in the vessel, which, 
moreover, would not contain more vapour than in the first 
case. The elastic force of the vapour is the same in the 
first and the third cases ; it is less in the second. Thus, the 
elastic force of a vapour is said to have attained its maximum 
when the space which contains it is saturated, and saturation 



ELASTIC FORCE OF AQUEOUS VAPOUR. 



1*7 



Hiay be known to have taken place when we see the vapour 
in contact with an excess of its liquid. 

The maximum elastic force of the vapour of any liquid 
is so much the greater as the temperature is higher. The 
quantity of vapour capable of saturating a given space 
follows the same law. We have supposed above the ether 
to be at 6o° F. ; at 68° F. the difference between the levels 
of the mercury would have been about thirteen inches ; at 
1 04° F. the level of the mercury would have become six 
inches higher in the open branch of the tube than in the 
other, as the pressure of the vapour is there superior to that 
of the atmosphere. At 140 F. we should mark about thirty- 
eight inches instead of six, and so on. 

When the elastic forces are very great we reckon them by 
the number of atmospheric pressures they are equal to ; 
what is called the pressure of one atmosphere being capable 
of sustaining a column of mercury thirty inches in height. 
The following is a table of the elastic forces of the vapour 
of water as deduced by Regnault from his numerous experi- 
ments : — 



TABLE OF THE ELASTIC FORCE OF SATURATED AQUEOUS 
VAPOUR AT VARIOUS TEMPERATURES. 



Pressure 
in Atmo- 
spheres. 


Temperatures. 


Pressure 
in Atmo- 
sphe es. 


Temperatures. 


Cent. 


Fahr. 


Cent. 


Fahr. 


I 

2 
3 

4 

5 
6 

7 
8 

9 
10 


IOO° 
121 

134 
144 
152 

159 
165 
171 
I76 
l80 


212° 

249 
273 
291 
306 
318 
329 

339 
348 
356 


II 
12 

13 
14 

15 
16 

17 
18 

19 
20 


185 

188 

192 

I96 

199 

202 

205 

208 

2IO 

213 


364 
371 

377 
384 
39o 

395 
400 
405 
410 
4i5 



N.B. The decimal fractions are omitted. 



i8S 



THE PHENOMENA AND LAWS OF HEAT. 



We meet with a very simple domestic application of this 
property of vapour in Papin's digester. This apparatus is 
composed of an iron or copper vessel (fig. 70), having very 
thick sides, which can be closed hermetically with a cover 
of the same metal by making use of a pressure screw. This 




Fig. 70. — Papin's Digester. 



cover has a hole, which is closed by resting a lever on it, 
on which is hung a weight. Water is put in this vessel, 
and it is then placed over the fire. Vapour forms on the 
surface of the water and mixes with the air inclosed in the 
interior; by removing the lever for some moments the mix- 
ture of air and vapour is allowed to escape, and soon nothing 
remains in the vessel but a mixture of vapour and liquid. 
The lever is replaced, and thus the conditions are obtained 
of a space empty of air which contains aqueous vapour with 



PRINCIPLE OF PAPIN'S DIGESTER. l8q 

an excess of liquid. The vapour formed presses its en- 
veloping walls on all sides ; its elastic force increases 
rapidly with the temperature, as the preceding table indi- 
cates. It is soon sufficiently great to raise the lever and to 
throw out a noisy jet, which forms a cloud. A weight is 
added to the end of the lever to close the digester, and the 
jet ceases ; the temperature, constantly rising, the elastic 
force continues to increase, and the lever must be still 
further weighted if we wish to prevent the escape of vapour. 
A temperature of 415 F. having been reached, supposing 
liquid water still to remain in the vessel, the vapour will have 
attained an elastic force of twenty atmospheres ; in this case 
supposing the surface of the cover to measure six square 
inches, it will receive a pressure of about 1,800 lbs. Such 
an experiment is of course not without danger, as the vessel 
would necessarily explode if it were not sufficiently strong. 
This remark will enable us to appreciate the immense diffi- 
culties that the researches of Regnault must have presented. 

Papin's digester has received an important application. 
As it enables us to have water in the liquid state at a tem- 
perature much above the ordinary boiling-point, and certain 
substances, such as the gelatine of bones, are dissolved so 
much the more easily by water as its temperature is higher, 
this digester may be used to obtain solutions of these sub- 
stances. By subjecting fresh bones to the action of this 
apparatus, the gelatine is abstracted, and from its use in 
this respect the name of digester has been derived. 

It is said that the table of one of the French prefects has 
been served for forty years past with gelatine thus extracted 
from fossil bones, which must have been buried in the soil 
for many thousands of years. 

We have considered above the influence of an elevation 
in temperature on evaporation in a vacuum ; it is evident 
that a fall would act in an inverse manner, so that the maxi- 
mum elastic force of a vapour has always the same value as 
the vapour passes a given temperature, whether it be in the 
act of becoming hotter or colder. For example, at 250 F. 
the elastic force of the vapour of water, when in contact with 
an excess of water 3 is always two atmospheres : this is a 



190 THE PHENOMENA AND LAWS OF HEAT. 

determinate state of saturation ; it is therefore impossible foi 
aqueous vapour to have, at one and the same time, a tem- 
perature below 250 F. and an elastic force of more than 
two atmospheres. An analogous remark might be made for 
each degree of saturation. 

To resume : there are many liquids, and some solids, 
which cannot exist freely in a vacuum without the molecules 
on their surface separating from each other and passing into 
the state of gas. The number of molecules which undergo 
this transformation depends on the nature of the substance 
and on its temperature ; it is greater as the temperature rises. 
When the quantity of gas has become sufficient, it exerts a 
certain pressure on the surface of the substance, and evapor- 
tion ceases. This property depends on the state of the 
molecules and on their mutual actions ; in the interior of a 
body, each molecule is under the influence, on every side, of 
other molecules ; it is therefore less free than it would be in 
a free state, to yield to other forces opposed to this action, 
and it is not astonishing that it behaves differently when 
under the influence of heat. Heat reigns over the condi- 
tion of each molecule, and consequently also the tempera- 
ture of the body ; it acts as an expansive force, opposed to 
the molecular attractions, and it is supposed to be in equi- 
librium in the interior, whilst it predominates on the surface. 
In the parts under its domination its effect is entirely to 
destroy cohesion and to produce that which we have already 
often called an interior work. In the preceding chapter an 
analogous difference has been shown to exist between the 
interior and the surface of a solid body relatively to fusion. 
As to the cessation of evaporation, it is due to the pressure 
exerted by the vapour formed, which is a contrary force to 
the expansive force of heat, and hinders this last from 
further overcoming the force of cohesion. 

To confirm our theory it will only be necessary to prove 
that evaporation is accompanied by the disappearance ot 
sensible heat, of which we shall indeed find numerous ex- 
amples in the following chapter. But we must first con- 
tinue our study of the circumstances under which evapora- 
tion takes place. 



VAPORISATION IN GAS. IQ1 

Again making use of the apparatus (fig. 69) already de- 
scribed, we introduce an excess of ether into the reservoir, 
allowing the air to remain. The column of mercury in the 
tube will be seen slowly to be displaced. Vapour is there- 
fore slowly formed, and mixes with the air. After some 
time the level of the mercury remains stationary, showing 
that the air is saturated with vapour. By the position of 
these levels it is concluded that the quantity of vapour pro- 
duced is the same as if the liquid had been introduced into 
a vacuum. 

We infer from this experiment, that a liquid evaporates in 
a space containing air, or, to speak more generally, a gas 
which does not act chemically upon it, as if this space were 
empty. The only difference is in the rapidity of the pro- 
cess. The production of vapour is instantaneous in a 
vacuum, and takes place slowly in a gas. The slowness of 
evaporation in the latter case is naturally explained by the 
mechanical obstacle which the gas presents to the separation 
of the molecules of vapour. At first they remain accumu- 
lated on the surface of the liquid, and keep together the 
superficial molecules which tend to separate, gradually ex- 
tending as their elastic force overcomes the resistance of the 
gas. In the process of evaporation we have, therefore, 
under consideration an exterior mechanical work, that which 
effects the displacement of the gas ; and the interior work, 
which effects the separation of the molecules on the surface 
of the liquid. But the air or gas acts only by the inertia 
of its mass, and the vapour alone averts the evaporation. 

We have now to consider the more ordinary phenomenon 
of evaporation in the atmosphere : the air can only become 
saturated in the immediate neighbourhood of a volatile 
body, because of the immense extent of the gaseous en- 
velope of the earth. As the same atmospheric strata do 
net long remain in contact with the body, the latter yields 
fresh vapours to each rapidly succeeding arrival of fresh air, 
and soon ends by being entirely evaporated. Its molecules 
remain in the state of gas, mixed with the air, until cir- 
cumstances permit it again to condense. For example, in 
winter, a dry wind causes the disappearance of ice and snow, 



192 THE PHENOMENA AND LAWS OF HEAT. 

without fusion taking place, because it continually removes 
the vapour of water produced by evaporation, and thereby 
renders the latter very active. It is in consequence of this 
that wet linen can be dried in very cold weather, and even 
though it be fiozen. Similarly in summer-time, a wind 
happening to come during the night, causes the dew already 
deposited to disappear. We constantly see the effects of 
the evaporation of water and other volatile substances. 
The more elevated .he temperature, the more marked are 
these effects. In the equatorial regions, the vapour of water 
rises above the seas heated by a burning sun ; coming in 
contact with cold air, it condenses into small drops, which 
form clouds ; these drops, rising higher, carried by the as- 
cending current disappear in the strata of dry air; and the 
phenomenon which engaged our attention at the commence- 
ment of this chapter, is presented to our notice on an 
immense scale. Very often we are able to watch the slow 
disappearance of a cloud over our heads : the mist of the 
morning is dissipated in the middle of the day, either 
because it has risen and formed clouds, or because the solar 
rays have evaporated it ; and in this case, the sky may 
remain clear. Side by side with this triumph of heat, we 
meet with the triumph of the molecular forces ; the return 
of clouds, their transformation into rain, snow, hail, pro- 
claims the reunion of the molecules of water. Leaving the 
earth, they have accomplished an immense aerial voyage, and 
have returned to undergo even more marvellous transfor- 
mations, as we shall shortly see. But let us follow our study 
methodically, and see if we cannot prove that vaporisation 
consumes heat. 

3. Evaporation is accompanied by a disappearance of 
sensible heat. 

When a liquid evaporates near a source of heat, it is so 
easy to conceive that the heat transmitted to its surface 
does the work necessary to effect the change in state, that 
we need not trouble to make a conclusive experiment on 
this point. It will be more to the purpose if we devote 



VAPORISATION CONSUMES HEAT. I 93 

our attention to the cases of evaporation, in which there is 
no apparent source of heat. 

Pour a little ether on your hand ; it evaporates, and you 
feel considerably cold : with certain liquids, such as the 
sulphide of carbon, which is less volatile than ether, the 
cold would be less, and it would be less again with water, 
which is still less volatile. There is then some relation 
between the quantity of vapour formed, and the amount of 
sensible heat that disappears. This remark will have sug- 
gested to the careful reader that the interior work of vapo- 
risation consumes a quantity of sensible heat taken from the 
non-evaporated portion and the neighbouring bodies, and 
that this heat is proportional to the work. An idea of the 
amount of effect produced may be formed by enveloping 
the bulb of a thermometer in a piece of linen, and soaking 
the linen successively with the several liquids above-men- 
tioned. By moving the thermometer about so as to favour 
an active evaporation, the full temperature in each case may 
be measured. The result will be exactly the same as that 
deduced from the sensation of cold felt in the first experi- 
ment. With ether, the temperature will fall several degrees 
below the freezing-point of water; with water it will fall a 
few degrees only below the ordinary temperature. It is easy 
to understand that the amount of disappearance of heat, 
and the quantity of vapour produced in similar experiments, 
may be measured, and their proportion verified. 

If vaporisation consumes heat, the condensation of vapour 
should, reciprocally, create heat ; for, clearly, this conden- 
sation represents the expenditure of a molecular work, arid 
we know that such an operation is habitually accompanied 
by the production of heat. We shall presently find some 
proof of this assertion. Indeed, do not certain every-day 
operations, which escape no one, already provide us with 
one ? The temperature of the air, for instance, is consider- 
ably milder in winter after rain ; and how is it that this rain 
produces heat, if it is not because the atmospheric water 
vapour is condensed into liquid drops ? 



N 



134 



THE PHENOMENA AND LAWS OF HEAT. 



4. Ebullition under a constant pressure, — Law of Temperature 

A third method of transforming liquids into vapour is 
that of ebullition, or the vaporisation of a liquid accompanied 
with the formation of bubbles of vapour throughout its 
mass. The investigation of this phenomenon may be made 

with the aid of very simple 
experiments, and it will com- 
plete our study of evaporation. 
Let us ascertain the progres- 
sive action of heat on water 
contained in an open vessel 
such as a glass flask. To do 
this, we will place the flask 
on a brazier, and immerse a 
thermometer in it, and ob- 
serve what passes in the liquid 
(fig. 71). The temperature 
gradually rises, and ascend- 
ing and descending currents 
spread the heat by convection ; 
vapour is formed at the surface, 
which condenses on issuing 
from the flask as a light cloud. 
These phenomena have al- 
ready been studied. Soon, 
little gaseous bubbles are pro- 
duced in the middle of the 
water, and rise slowly to the 
surface; these bubbles con- 
sist of dissolved air. Then 
larger bubbles appear at the bottom from various points of 
the glass; as they rise, they diminish in volume and dis- 
appear, without reaching the surface; a noise is then heard 
which is universally known as singing. The explanation is 
as follows. Each bubble consists of the vapour of water, 
and this vapour is produced around a little bubble of air, 
when its temperature has risen near the boiling-point 
(ioo° C. or 2 1 2 Fahrenheit). As the flask is heated from the 




Fig. 71. — Ordinary Ebullition. 



TEMPERATURE OF EBULLITION. 1 95 

bottom, the lower parts of the liquid attain this temperature 
before the upper parts, and the vapour, formed as it rises, 
comes in contact with water less heated than itself; it cools 
in giving up heat to this water, and condenses suddenly ; a 
small vacuum exists for a moment in its place, and the sur- 
rounding water rushes into it with a shock ; thence a trepi- 
dation of the liquid and a noise. At length the bubbles of 
vapour are able to reach the surface, the thermometer marks 
ioo° C. and the liquid is in full ebullition. Bubbles may be 
seen to become larger as they rise and burst in the air in 
the act of raising a thin pellicle of water of a hemispherical 
form. 

The first fundamental law of ebullition is the constancy 
of its temperature, on condition that we operate as above. 
Each liquid has a fixed boiling-point in free air, the same as 
each solid has its temperature of fusion. Thus water boils 
at ioo° C, alcohol at 79 C, ether at 36 C. 

It is this constancy of temperature which establishes the 
difference between evaporation and ebullition; but these 
two phenomena are only different forms of vaporisation or 
passage from the liquid state to the gaseous. In each the 
change of state is effected on the free surface of the liquid, 
conformably to the principles we have laid down ; only, in 
evaporation it is the visible surface which gives the vapour, 
and in ebullition there are an indefinite number of little 
surfaces enveloping microscopical bubbles of air, whether 
against the sides of the vessel, or in the ii terior of the liquid 
mass. It is on these little surfaces that vaporisation is 
effected. Really, ebullition is only evaporation operating 
on a large number of points at once. 

Many experiments may be made, which prove that ebul- 
lition is due to the presence of bubbles of air or other gas. 
The following are two very curious ones. The first, made 
by Donny, consists in inclosing water in a bent glass tube, 
as represented in fig. 72. This tube being open, and drawn 
out to a smaller diameter at one of its ends, the water is 
made to boil for some moments, so as completely to expel 
the air dissolved in the water and that contained in the rest 
of the tube. Whilst the tube contains vapour and water 



iq6 



THE PHENOMENA AND LAWS OF HEAT. 



only, the point is closed by melting it in a gas flame ; it ia 
allowed to cool, and the apparatus is finished. When it is 
to be used, the part which contains the liquid is placed in 
an oil bath, which is heated by means of a lamp. By placing 
a thermometer in the oil, we shall see that the temperature 
may rise to 130 C. without boiling the water. But at about 




CP 



Fig. 72. — Donny's experiment in Ebullition. 

this temperature the water is thrown entirely into the other 
part of the tube, producing a shock which is without danger, 
in consequence of the form given to this part. 

The second experiment is that first made by M. Dufour, 
of Lausanne, and it is still more demonstrative. A drop ot 
water is allowed to fall into a mixture of oils at the temper- 
ature of ioo° C. and of which the proportions are so calcu- 
lated that its density is the same as that of water at the 
same temperature. The water forms a spherical globe in 
the middle of the liquid, and maintains this form whilst the 
temperature is made to rise several degrees above ioo° C. : 
there is no ebullition, because the drop of water being sur- 
rounded on all sides by the liquid, has no surface for evapo- 
ration. If the drop be touched by a stick of wood, bubbles 



EBULLITION UNDER VARIOUS PRESSURES. I97 

of vapour immediately appear at the point of contact ; the 
reason being that the stick of wood has carried with it some 
bubbles of air which have come in contact with the water : 
evaporation has thus become possible, the vapour diffuses 
into each bubble of air, and enlarges it, giving it such a 
size that it detaches itself from the wood, and rises to the 
surface of the oil. 

There is a second fundamental law of ebullition which 
bears evidence to the exterior resistances to vaporisation. 
We have said that water boils at too C. This cannot take 
place unless the height of the mercury in the barometer is 
thirty inches. Now this condition is only accidentally filled 
in countries but little elevated above the level of the sea, 
and it never is on high mountains. At the top of Mont 
Blanc the mean height of the barometer is about 16J inches. 
Under this pressure water boils at 84 C, and in the same 
circumstances ether, which boils at 3 6° C. under the ordinary 
pressure, boils at so low a temperature as 20 . Generally, 
the less the atmospherical pressure the lower is the point of 
ebullition in the open air. The reason of this law is easy 
to understand : each bubble of vapour in its formation has 
to raise and overcome the resistance of a surrounding liquid. 
Now the pressure that the atmosphere exerts on the surface 
is transmitted by the liquid to the bubble of vapour; and if 
the depth of the liquid is small, this pressure is the principal 
cause of resistance. The vapour has to overcome this 
resistance, and in reality with a somewhat superior expan- 
sive force ; because it is only when it reaches the atmo- 
sphere, there to spread itself abroad, that the two pressures 
become equal. The bubbles of vapour, therefore, begin to 
form when the liquid attains the temperature at which its 
vapour possesses a maximum elastic force equal to the 
atmospheric pressure. We have already seen that this tem- 
perature decreases in the same ratio as the elastic force of 
the vapour. The effects observed on Mont Blanc are thus 
explained. 

So soon as ebullition commences, the temperature remains 
invariable, subject to the condition that the pressure does 
not change. If the pressure augmented on the surface of 



I98 THE PHENOMENA AND LAWS OF HEAT. 

the liquid, ebullition would stop for a moment, until the 
liquid had acquired sufficient heat from the source to raise 
its temperature to the necessary point. The table on page 
187 gives the temperatures at which water boils under various 
pressures 5 for example, if it be submitted to a pressure 
of two atmospheres, its temperature must rise to 121 C. 
or 2 49 F. before it begins to boil. In the steam-engine 
ebullition takes place at a constant temperature, when the 
lire is managed in such a manner that the quantity of vapour 
developed in the boiler is constantly equal to that which 
leaves it to act in the cylinder ; it is on this condition that 
the pressure continues invariable in the boiler, and that the 
work of the machine is regular. Our table shows the rela- 
tion which exists between this pressure and the boiling-point 
of water. If the engine is under a pressure often atmo- 
spheres, the water boils at 180 C. or 356 F. Suppose the 
careless stoker should overcharge the furnace with coal, the 
temperature will rise, because the quantity of vapour formed 
will be more than that which is able to leave the boiler, and 
it will therefore cause the pressure to be rapidly augmented. 
At 202 C. or 395 F. the pressure would be sixteen atmo- 
spheres, and at 213 C. or 415 F. would be as much as 
twenty atmospheres ; so that in allowing the temperature to 
rise 30 ° C. or 54 F. only, the stoker will have doubled the 
pressure. Explosion might result from this unless the 
machine were able to bear so immense a pressure. It is 
for this reason that the boiler is furnished with safety valves, 
which allow the vapour to escape so soon as the pressure 
becomes too great. 

5. Interior and exterior work. — Heat of evaporation. 

What part does heat play in ebullition when the pressure 
remains constant, and consequently there is no change in 
temperature ? Evidently, the heat ceaselessly furnished by 
the stove is no longer sensible after passing into the liquid, 
inasmuch as the thermometer does not give evidence of its 
presence ; it has been consumed, annihilated as heat, but its 
equivalent will be found in the mechanical work produced 



HEAT OF VAPORISATION. TQ9 

We must here take into consideration both the interior 
work due to the liquid molecules being separated in spite 
of the cohesive force which unites them, and the exterior 
work due to the volume or bulk being increased in spite of 
the resistance of bodies which press on its exterior surface. 
When the liquid boils in free air, it is the atmosphere which 
offers this resistance. When it boils in the steam-engine, it 
is the piston. In these circumstances, the exterior work is 
quite comparable in amount with the interior work, and it 
can never be neglected as in the fusion of solid bodies. 
This depends on the fact that the bulk of the vapour is 
always much greater than that of the liquid from which it is 
produced. Thus water in vaporising at ioo° C. expands to 
a volume 1,700 times its former bulk. 

To represent the work produced, imagine a cylinder one 
square decimetre in section containing a kilogramme of 
water at ioo° C. and a piston exerting on the surface of the 
water a pressure of 103 kilogrammes. When 536 thermal 
units shall have been spent, this water will have been reduced 
into vapour, and the piston will have been raised about 170 
metres. The exterior work produced is then more than 
17,000 kilogrammetres, and it has consumed 40 thermal 
units ; that is to say/ nearly one-thirteenth of the whole 
quantity of heat expended. 

To determine the heat of vaporisation, — that is to say, the 
number of thermal units consumed by a liquid when it is 
reduced into vapour under constant pressure, — we must 
measure the heat disengaged by the vapour when it resumes 
the liquid state under similar circumstances. To illustrate 
this, remove the source of heat from the cylinder filled with 
vapour, which has served us in the above demonstration. 
The result will be that the piston will redescend as fast as 
the molecules of vapour now disenthralled obey their mutual 
attractions and reconstitute the liquid. There will be thus 
two kinds of work spent : first, that of the piston, and after- 
wards that of the molecular forces ; and hence so much 
heat. disengaged as may be equivalent to the whole of this 
work. When the kilogramme of water at ioo° C. has be- 
come reconstituted, the total amount of heat disengaged 



200 THE PHENOMENA AND LAWS OF HEAT. 

will be exactly equal to that which had been expended in 
the vaporisation. The condensation of vapour effected 
under the same conditions of pressure as ebullition, pre- 
sents the same relations between heat and work ; there is 
simply inversion in the sense of these quantities. Now it 
is very easy to measure the heat disengaged : to effect this 
it suffices to surround the cylinder with cold water. From 
the weight of this water and the elevation of its tempera- 
ture, the number of thermal units may be calculated. This 
is the reason that it is preferred to make the estimation in 
the return to the liquid state ; and it is by making experi- 
ments of this kind that Regnault has found that in the 
operation just described 536 thermal units are expended. 

Distillation furnishes a very simple example of the con- 
densation of vapours under a constant pressure. This 
operation effects the separation of a liquid mixture into its 
constituents of unequal volatility, the more volatile coming 
over first. The simplest case is that of a simple liquid in 
which non-volatile matters are dissolved or held in suspen- 
sion. The mixture is introduced into a boiler presenting a 
large surface to the fire. Ebullition takes place; vapour 
arises from the liquid only ; it passes into a serpentine tube 
surrounded by cold water and open to the air at its ex- 
tremity, so that the atmospheric pressure is freely exercised 
within the apparatus (fig. 73). The cooled vapour con- 
denses at first under this pressure, disengaging heat but keep- 
ing its temperature. The liquid thus produced descends in 
the serpentine, gradually cooling, and continues to disengage 
heat until it has attained the temperature of the water ; it 
is collected in a vessel placed below the apparatus. As to 
the non-volatile matters, they accumulate in the boiler, being 
completely separated from the liquid. 

The heat disengaged during the condensation of the 
vapour is employed in heating the water which surrounds 
the serpentine tube ; and as it is necessary, to prevent loss 
of vapour, that the distilled liquid should issue as cold as 
possible, the water, as it becomes warm, must be replaced 
by fresh cold water. This enters from below, and the hot 
water runs out from above. Such an apparatus is termed a Still 



PRINCIPLE OF THE STILL. 



20I 



It must be remarked that the heat disengaged in the worm 
is equal to that received from the fire, the mixture having 
once reached the boiling-point, and loss by conduction, &c, 
being overlooked. This distillation, therefore, effects a 
veritable transportation of heat. This operation has been 
applied for heating purposes. The process of heating by 
the circulation of steam is nothing more than distillation on 
a large scale. Suppose a house is to be heated, the boiler 




Fig. 73.— Still. 



is fixed in one of the cellars, and pipes are arranged in the 
form of an immense serpentine throughout all the floors of 
the house, along the walls, and under the floors ; the steam 
rises into them, condenses, disengages heat, and the con- 
densed water returns to the boiler. It again becomes heated 
and circulates afresh ; in the same manner that the blood 
receives heat from the lungs, distributes it in all parts of the 
body, and returns to the source to repair its losses. 



202 THE PHENOMENA AND LAWS OF HEAT 

6. The utilization of Heat in the steam-engine and 
hot-air engine. 

A steam-engine furnished with a condenser might be 
compared to a still, if it were only regarded superficially. 
There is the boiler, in which the water spends heat to effect 
its reduction into vapour, and the condenser, a reservoir 
surrounded by cold water, into which the vapour enters 
after leaving the cylinder ; the water here again assumes 
the liquid state, disengaging heat ; and it might be sup- 
posed — as, indeed, was for a long time thought — that the 
heat disengaged in the condenser is equal to the heat 
expended in the boiler. It has been proved, however, by 
numerous experiments, especially those of Hirn, to which 
we have alluded in our first chapter, that this equality does 
not really exist. The heat disengaged is always less than 
the heat expended, and the difference is proportional to 
the mechanical work effected by the piston of the machine, 
so that the heat taken from the fire by the water in the 
boiler is partly transported into the condenser, where it 
heats the surrounding substances, and partly annihilated 
as heat, and transformed into mechanical motion in the 
cylinder, where an exterior work is effected. The most 
perfect machine is that in which the proportion of heat 
transformed into work is the greatest possible ; and theory 
indicates that this proportion can scarcely exceed one-sixth 
of the heat really absorbed by the steam. In other words, 
taking the most perfect steam-engine known, and supposing 
that six thermal units are supplied by the fire to the boiler, 
one unit is converted into work, and five are transported 
to the condenser, as in a still. 

On comparing from this point of view the steam-engine 
with the hot-air engine, of which we have given an idea in 
our first chapter, it is obvious that the latter is, theoretically, 
to be preferred. For example, when the air of the machine 
is heated but a little above 300 C, which limit is necessary 
to prevent the parts from being destroyed by rapid oxy- 
dation, it is possible in theory to convert half the heat 



SUPERIORITY OF THE HOT-AIR ENGINE. 203 

expended into mechanical work, the other half being 
simply transported into the surrounding bodies, — a result 
much superior to that given by the steam-engine. The 
construction of air-engines ought, therefore, to be seriously 
studied by inventors, with a view to their mechanical per- 
fection • for, unfortunately, the numerous attempts that 
have been made to construct large air-engines have been 
far from realising the hopes that were justly founded upon 
the advantages they offer in principle. Hitherto, in conse- 
quence of their bad construction, and in many cases because 
the principle appears not to have been grasped, hot-air 
engines have not exhibited any practical superiority over 
steam-engines. The future may show a very different 
result. Hot-air engines are in no danger of exploding ; 
they can be worked without water, and, above all, they 
effect the conversion of heat into work in the most eco- 
nomical manner. Inventors need not lose courage. With 
the example of the illustrious Watt always before them, 
they should rather apply themselves to master thoroughly 
the laws which have been placed within their reach by the 
new theory of heat. Was it not, after unheard-of efforts 
and immense pecuniary sacrifices, that the inventor of the 
steam-engine arrived at his end ; and how many had pre- 
viously succumbed ? Enough to mention here that Denis 
Papin, of Blois, discovered the principle of this machine in 
the seventeenth century, a discovery which has immortalized 
his name, though its application to the industry of the world 
was the achievement of others. 

To return to the phenomena of the steam-engine. It is 
an experimental fact that a kilogramme of steam, by lique- 
fying in the condenser, disengages a quantity of heat less 
than that which had been expended in its formation in the 
boiler. We can explain this by examining the conditions 
under which the vaporisation of the water and the liquefac- 
tion of the vapour are effected. The water is converted 
into vapour under a constant pressure, and the heat ex- 
pended serves, on the one hand, to separate the molecules 
of the liquid, thus effecting an interior work ; while, on the 
other hand, it surmounts the resistance of the piston, and 



* 04 THE PHENOMENA AND LAWS OF HEAT. 

thus effects an exterior work. The total heat expended is 
equivalent to the sum of these two works. In the inverse 
operation, the vapour issues from the cylinder into the 
condenser, where the pressure is very feeble ; the exterior 
work expended during the diminution of the bulk of this 
vapour is not considerable, and creates little heat; the 
interior work spent by the molecular forces at the moment 
of liquefaction is alone capable of effecting the disengage- 
ment of a quantity of heat comparable with the heat spent. 
It is, therefore, the absence of a sufficient exterior work 
during condensation which explains the facts observed by 
Hirn. 

We thus see that a vapour when condensing under a 
constant pressure, or under a gradually decreasing pressure, 
does not disengage the same quantity of heat that had been 
absorbed in its formation, and this observation leads us to 
study ebullition under analogous circumstances. 

7. How a liquid may be made to boil by cold. 

It is not necessary that a liquid should be in contact with 
what is commonly called fire in order to boil. There are two 
conditions necessary and sufficient for ebullition : first, that 
the temperature of the liquid be below the surrounding 
bodies; secondly, that the pressure exerted on its open sur- 
face be less than, or at most equal to, the maximum elastic 
force possessed by the vapour at the temperature under 
consideration. In fact, around each little bubble of air 
contained in the liquid, vapour tends to form as soon as it 
is reached by the sensible heat from the surrounding bodies, 
and it is hindered in its formation by the pressure exerted 
on the bubble of air by the surrounding liquid : a pressure 
which results from the action on the parts comprised 
between the bubble and the surface, and from the resist- 
ance of the air or gas situated above this surface. The 
state of the liquid molecules under the double influence of 
heat and of pressure is comparable to that of a stretched 
spring. If the obstacle which holds it offers a limited 
resistance, it will be overcome by a gradually increasing 



EBULLITION IN VACUO. 



20S 



tension, and the spring will relax itself suddenly. Similarly 
the molecules (of water, if we please) are gradually heated 
by neighbouring bodies, and this heating continues so long 
as their temperature is lower than that of the surrounding 
bodies, and there arrives a moment when they separate, 
surmounting the exterior resistance ; obviously they possess 




Fig. 74— Ebullition in vacuo. 



at this critical moment an elastic force at least equal to the 

resistance they overcome. 

This reasoning shows that ebullition is not always effected 
under a constant pressure and an invariable temperature, as 
in the cases with which we have as yet been occupied. 

Connect a little glass flask containing ether with a large 
reservoir by means of a leaden pipe (fig. 74). This reservoir 
is closed by a stop-cock, and the air which it contained has 
been removed by the air-pump. Open the communication, 
and we shall soon see the ether boil with as much activity 
as if we had held the flask over the fire. If a thermometer 



206 THE PHENOMENA AND LAWS OF HEAT. 

has been placed in the liquid, we shall see at the same time 
that the temperature has fallen several degrees. In fine, 
when the ebullition has lasted a little time, it will entirely 
cease. This observation confirms the accuracy of our 
reasoning. 

Before opening the communication, the ether is at the 
ordinary temperature, and its surface is pressed by the 
atmosphere. After communication is made with the empty 
reservoir, the air in the flask expands, and diffusing itself 
through the greater space, its pressure becomes compara- 
tively slight. The molecules of ether then become vaporised 
at the surface of the liquid, because the pressure of the 
atmosphere has been removed. Vaporisation being effected 
by the production of interior work, a quantity of sensible 
heat equivalent to the work disappears, and in consequence 
the temperature of that portion of the ether which has 
remained liquid falls. It will now be colder than the sides 
of the flask and other bodies near it, and consequently these 
act upon it, as if they were sources of heat : ebullition can, 
therefore, continue so long as the pressure remains suffi- 
ciently small. But the vapour, accumulating slowly in the 
reservoir, forms in conjunction with the air a mixture the 
pressure of which is constantly increasing; this pressure 
offers to the surface of the ether a resistance which is also 
increasing, and this soon becomes sufficiently great to stop 
the ebullition. At that moment the -resistance equals the 
maximum elastic force of the ether vapour at the tempera- 
ture of the flask. 

If we cool the reservoir by surrounding it with ice, or 
otherwise, ebullition will again commence in the flask, 
because the ether contained in the reservoir is partly con- 
densed by cooling, and the pressure diminishes in the 
interior. It has, therefore, ceased to obstruct the formation 
of vapour in the flask. As fresh vapour is disengaged it 
enters the reservoir and is condensed, and the ebullition 
may then continue. A true distillation is thus effected 
without any apparent application of heat, but there is no 
essential difference between this phenomenon and that 
exhibited by the Still. This distillation may, moreover, be 



EBULLITION BY COLD. 



207 



effected with a constant temperature of ebullition if the 
pressure is maintained invariably in the apparatus. Thus 
the fact that liquid may be made to boil by cold, which 
appears singular when thus simply announced, is easily 
explained. 

The experiment may be made with water in the fol- 
lowing simple manner, without the need- of expensive 
apparatus: — First, water is 
made to boil in a long- 
necked flask (fig. 75), so 
that the steam arising from 
it shall completely fill the 
flask to the exclusion of 
air. It is then corked 
tightly, turned up, and the 
neck is plunged into water, 
so as entirely to prevent 
the entrance of air through 
the interstices of the cork. 
The flask is thus in a cool- 
ing condition, so long as 
its temperature remains 
higher than that of the 
surrounding bodies. The 
vapour over the liquid 
partly condenses, and there 
remains only a sufficient 
quantity to keep the pres- 
sure great enough to pre- 
vent further vaporisation. 
This being the state of things, we can make the water boil 
again by pouring cold water on the flask ; for the vapour 
being thus rapidly condensed, its pressure suddenly dimin- 
ishes, as if a vacuum had been produced over the 
liquid, which is immediately seen to boil. When the appa- 
ratus has cooled to the ordinary temperature, the water it 
contains may be again made to boil by applying a piece of 
ice to the upper part, or by wetting it with a little ether, 
which, by its rapid evaporation, produces cold like ice. 




Fig. 75. — Ebullition of water by cold. 



2o8 THE PHENOMENA AND LAWS OF HEAT. 

The decrease of interior pressure thus occasioned again 
causes ebullition of the liquid. 

To resume : the vaporisation of a liquid is effected under 
the following circumstances : — First, by evaporation in a 
vacuum, or in a glass, until the vapour formed from the sur- 
face has attained a fixed maximum elastic force, depending 
on the temperature ; secondly, by ebullition at a constant 
temperature when the pressure is itself constant, and this 
temperature is below that of the surrounding bodies ; thirdly, 
by ebullition in a vacuum or in an artificial atmosphere of 
gas the pressure of which is less than the maximum elastic 
force of the vapour which is relative to the temperature of 
the liquid. 

All liquids are not susceptible of being transformed into 
vapour by ebullition. Mercury may be made to boil at 
360 C. or 68o° F., but the less fusible metals, such as gold 
and platinum, would require temperatures so high as to be 
unattainable. All that we can say of these bodies is, that 
they are volatile at very high temperatures : farther, many 
liquids are decomposed by heat, and their vaporisation is 
consequently very difficult; their distillation often impossible. 
In these cases, above all, recourse is had to distillation in 
a vacuum, because a very low temperature, at which decom- 
position does not take place, suffices for the effect. This 
method is frequently utilized in chemistry. 

8. The Geysers. 

Let us now try to apply the principles we have explained 
to those impressive natural phenomena which have often 
been attributed to mysterious causes, and which have so 
long exercised the sagacity of savants. 

Iceland is a volcanic island, in which a chain of moun- 
tain rises covered with eternal snows. A dozen volcanoes 
are distributed along this chain, the best known of which 
is Hecla. Glaciers descend from the snowy summits and 
give rise to immense cataracts of water, which spread out at 
the foot of the mountains and cover spaces of considerable 
extent, forming vast marshes, the bottom of which is split 



THE GREAT GEYSER. 200 

np by volcanic action. The water becomes engulphed in 
these crevasses, and through subterranean channels pene- 
trates into the heart of the mountains, where it becomes 
heated, and subsequently issues from the craters in torrents 
of steam. From time to time we meet with smoking lakes 
and quagmires, upon the surface of which immense bubbles 
rise, which in breaking throw their froth several yards into 
the air. In other places there are intermittent jets of boil- 
ing water, called Geysers. Everywhere, and under various 
aspects, the action of the central heat is made manifest, ap- 
parently as if Nature had gathered together in this desolate 
place her most terrible engines of destruction. 

But forget the noise of the explosions, approach a geyser 
whilst at rest, and you will see a marvellous well, fashioned 
by the hand of Nature herself. Let us take the Great Geyser 
as our example. At the top of a mound some 12 or 13 
yards in height will be found a beautifully enamelled basin 
more than 50 feet in diameter. In the centre of this basin 
is a well 74 feet deep, and 10 feet in diameter, the sides 
of which are lined with the same siliceous enamel as the 
basin. " Over the surface curls a light vapour," says Prof. 
Tyndall; " the water is of the purest azure, and tints with 
its lovely hue fantastic incrustations on the cistern walls/' 
Such is the appearance of the geyser in its quiescent state. 

When an eruption is about to take place, the well and 
basin fill with hot water ; the ground shakes, subterranean 
explosions are heard, and the water is violently agitated. 
Presently the water swells np in a boiling condition, and 
overflows the basin of the geyser. This is speedily suc- 
ceeded by the true eruption. An immense column of water 
and vapour is thrown into the air, and falls back into the 
basin : some further detonations are heard, and then all is 
at rest. But this rest is only momentary ; the eruptions 
succeed each other with similar phenomena during several 
years, after which they cease. Nothing then remains of the 
geyser but the well, which continues to be supplied with hot 
water until it has found another issue. 

The celebrated German chemist, Bunsen, has attentively 
examined the Great Geyser, and he has not only explained 

o 



210 



THE PHENOMENA AND LAWS OF HEAT. 




Fig. 6.— Theory of Geysers. 



its action, but con* 
trived an experiment 
which reproduces the 
principal effects. 

The apparatus (fig. 
76) used for this ex- 
periment is made of 
a galvanised iron tube 
six feet long, set up- 
right and surmounted 
by a basin. This tube 
is filled with water, 
and heated at two 
points : first at the 
bottom by a furnace, 
secondly at a height 
of two feet from the 
bottom, by means of 
a circular grating. 
When the water has 
become sufficiently 
heated, it is ejected 
into the atmosphere, 
and, failing back into 
the basin, it refills the 
tube ; then a few 
slight detonations are 
heard ; and it is at 
rest. Soon, however, 
the same phenomena 
repeat themselves, 
and thus reproduce 
the intermittent ex- 
plosions of the natural 
geysers, which we will 
now briefly explain. 

The water at the 
bottom of the tube 
would boil under the 



PRINCIPLE OF THE GEYSER. 2 1 1 

pressure of the atmosphere, augmented by the pressure of 
the column of water, 6*6 feet in height, at a temperature 
of 105 C. or 221 F. The water situated two feet higher 
up the tube, having only to overcome the pressure of the 
atmosphere and a column of water four feet six inches in 
height, would boil at about 103 C. or 217 F. But if, 
when it is about attaining this temperature, it has no longer 
such a pressure to support, — for instance, if the column 
of water is removed, — it would be instantaneously converted 
into steam, its temperature falling to ioo° C. or 212 F., 
that being the boiling-point of water under the ordinary 
pressure of one atmosphere. Let us, therefore, conceive 
the water in the tube to be heated to nearly 103 C. or 
2 1 7 F. at the part next the circular grate, and that at the 
bottom to be entering on ebullition at 105 C. or 221 F. : in 
this case, the steam produced will raise the column of water 
throughout the length of the tube ; the basin will become 
filled, and that part of the water which is heated to 103 C. 
will be raised towards the upper part ; it will then support 
a column of water less than four feet six inches, its original 
height, and will be reduced into steam. This steam will 
completely drive the water from the tube, and in consequence 
of the suddenness of the effect the water will be projected 
above the basin as a jet of water mixed with steam. This 
jet will cool itself in the air, and then, falling back into the 
basin, will cool the steam remaining in the tube ; all the 
w r ater in the basin will rush into the latter as into a vacuum, 
with rather a violent shock ; some bubbles of steam might 
form at the point of contact with the hot sides of the tube, 
but they will be immediately condensed by contact with the 
cold water : these consecutive reactions will cause the little 
detonations before the resumption of rest. Sources of heat 
continuing to act will then re-establish the column of water 
in the same state as before, and all the phenomena of the 
eruption will be repeated. 

Bunsen has measured the temperatures of the water of 
the Great Geyser at various depths, and he has found that 
they decrease regularly from below upwards. The water at 
nine metres from the surface is only two degrees below the 



212 THE PHENOMENA AND LAWS OF HEAT. 

temperature of ebullition, which corresponds to the pressure 
supported by this part. It was therefore only necessary that 
it should be raised two metres to enable it to boil, and thus 
proj ect outwards the whole of the column of water above it. 
As to the cause of this lifting, it is due to the elastic force 
of the steam which arrives at the bottom of the well, con- 
ducted there by the subterranean wells from the volcanic 
depths where they are generated. 

This ingenious theory explains all the peculiarities of the 
geysers. The water which feeds them is charged with a 
siliceous matter, which is deposited on the sides of the basin 
as the water evaporates. The well or tube is thus gradually 
raised, together with the mounds on which it is situated, 
and lined with a deposit of siliceous enamel. In this manner 
the geyser must have been slowly constructed by a spring 
of siliceous water before the eruptions commenced. At first 
there would be simply a boiling spring, and the crystalline 
tube and basin would be slowly formed by the flowing over 
of the spring. The eruptions would first take place when 
the tube or well was deep enough to contain a column of 
water sufficiently high to obstruct the escape of the jet of 
vapour, although not preventing ebullition below. At length, 
in consequence of the augmentation in length of the tube, 
the column of water becomes sufficiently high to put an end to 
all ebullition ; the subterraneous vapour then seeks another 
outlet, and the geyser is extinct. 

9. The spheroidal state of liquids. — How the human body 
may be incombustible. 

The phenomena which occur when volatile liquids are 
placed in contact with very hot bodies are very remarkable, 
and it is but lately that they have been satisfactorily ex- 
plained. It is possible to plunge the hand into molten lead, 
touch molten type-metal, pass the tongue over red-hot iron 
without burning. The workmen in foundries are aware of 
these facts ; and recently, M. Boutigny, of Evreux, has made 
a special study of them, himself repeating these experiments. 
The hand must be carefully moi>tened with a very volatile 



SPHEROIDAL STATE OF WATER. 213 

liquid, such as alcohol or ether, when it is wished personally 
to prove these curious effects. Yet the natural humidity of 
the skin, especially when under the influence of some appre- 
hension, might suffice. It is also evident that the trial must 
be made rapidly and with considerable adroitness, simple 
radiation being sufficient to burn the parts of the hand next 
those which touch the molten metal. The momentary in- 
combustibility of the skin depends on the slight layer of 
liquid which moistens it : it is this which intercepts the 




Ftg. tj. — Spheroidal state of Water. 

passage of the heat. The explanation of this property of 
volatile liquids will, however, result from a less dangerous 
series of experiments that we shall be able to perform with 
inanimate matter. 

Heat a well-polished iron capsule to redness, and drop 
into it a few drops of cold wafer ; we shall see them collect 
together into a tremulous globule with rounded edges, which 
turns continuously on itself (fig. 77) ; there is no ebullition. 
no visible vapour, and yet the globule diminishes slowly. 
There is, therefore, slow evaporation over its surface ; but it 



214 



THE PHENOMENA AND LAWS OF HEAT. 



may be so slow, that Pouillet has kept for several hours a 
large platinum dish rilled with water. It is the vapour 
which, enveloping the waters on all sides, prevents immediate 
contact between the liquid and the hot surface. Escaping 
first from one side, then from the other, it breaks into the 
contour of the globule and makes it oscillate ceaselessly ; 
it acts upon it like an infinity of little hidden springs, 
alternately compressed and extended. Thence the fantastic 





Fig. 78.— Flame seen between the hot surface and the globuie. 



movements of the globule, which acts as if it were trying to 
escape from the fire, where it is retained by an invincible 
force. 

It is easy to prove the existence of a slight layer of 
vapour interposed like a little spring cushion between the 
incandescent surface and the globule. A horizontal plate 
of polished metal, silver for instance, is heated by means of 
a lamp (fig. 78). A drop of cold water being thrown on to 
the plate when sufficiently hot, takes upon itself the aspect 
we have just described. Having blackened the drop with 
a little ink, we may distinctly see the light of a candle 
through the thin line of vapour which lies between the 
underside of the globule and the silver plate. A beautiful 
exhibition may be made of this experiment by projection. 
By making a horizontal ray of light fall on the globule, and 
arranging a lens at the other side, a reversed image may be 



TEMPERATURE OF THE SPHEROIDAL STATE. 215 

shown on a screen, in which the plate of silver, the globule, 
the image of the globule in the plate, and the line of trans- 
parent vapour which separates them, are distinctly visible. 

The temperature of the water when in this peculiar state 
(which is called the spheroidal state) may be determined by 
the elevation in temperature produced in a known weight 
of cold water when the globule is thrown into this water ; 
or by placing the bulb of a thermometer in the midst of 
the globule — when it will be found that the temperature 
sought is always below ioo° C. ; it is therefore impossible 
for ebullition to take place. 

This fact is general : — Put ether instead of water in the 
heated capsule, and the temperature of the globule of ether 
will be below 3 6° C. ; it will neither boil nor inflame. 
Place in it some liquid sulphurous acid, which boils at ten 
degrees below zero ; the globule will be colder by several 
degrees. A few drops of water thrown on to this globule 
instantaneously congeals, thus producing ice in a red-hot 
vessel. This experiment might be made on a larger scale. 
Place in a platinum crucible strongly heated a large quantity 
of liquid sulphurous acid : on throwing some cold water into 
it, a considerable mass of ice may be obtained. Farady has 
even seen liquid carbonic acid, which is still more volatile 
than sulphurous acid, take the spheroidal state at ioo° below 
zero C, or 150 below zero R, and congeal an ounce of 
mercury in two or three seconds. 

This simply proves that volatile liquids, when placed in 
very hot vessels, cannot attain the temperature of ebullition, 
and that they only evaporate from their surfaces. 

In order for the spheroidal state to be assumed it is ne- 
cessary that the solid body be heated above a certain limit 
of temperature, which is special for each liquid, and which 
is lower in the degree that the liquid is more volatile ; ao. 
cording to Boutigny, the limit is 142 C, or 288 F. for 
water, and 61° C, or 142° F. for ether. If, after having ob* 
tained the spheroidal globule, the vessel be allowed to cool 
at the moment its temperature attains this limit, the liquid 
immediately touches it and boils with violence. The follow- 
ing is an experiment which demonstrates this fact in a very 



2l6 THE PHENOMENA AND LAWS OF HEAT. 

striking manner, and which possesses a peculiar interest^ 
because it illustrates one of the causes of explosion in steam- 
boilers. 

A copper bottle (fig. 79) is strongly heated; water is in- 
troduced into it, which takes the spheroidal state ; the bottle 
is corked and allowed to cool. When the temperature has 
fallen to about 142 C the water begins to boil and steam, 




FiG- 79. — Explosion produced by the Cooling of Water in the spheroidal state. 

developed in great quantity, almost instantaneously ejects 
the cork with explosion. 

Consider now the boiler of a steam-engine. When in a 
normal condition, its sides which are in contact with water 
are alone heated by the fire. The water, being thus kept at 
a constant temperature of ebullition, prevents the metallic 
sides of the boiler from becoming hotter. From any unex- 
pected cause, such as an incrustation which separates the 
water from the sides, they will become heated to redness ; 
and if the cause is then removed, if the incrustation presents 
fissures, the water will again come in contact with the incan- 
descent metal and will take the spheroidal state. When 
under these circumstances the heating is stopped, the tem- 
perature of the metal will fall, and an enormous mass of 



PRINCIPLE OF GLASS-BLOWING. 21 7 

vapour will be suddenly engendered at about 142 C. If 
the resistance of the boiler is not great enough to prevent 
it, there must necessarily ensue a terrible explosion. 

The same phenomenon is presented under another form 
by the immersion of an incandescent body into the midst 
of a cold liquid mass. Make a ball of metal, suspended 
by an iron wire, red-hot, and plunge it quickly into cold 
water ; a kind of crackling will be heard, and the ball will 
remain red for some time without the surrounding water 
becoming apparently heated : a sheath of vapour is around 
the ball which prevents contact. But the ball is presently 
cooled to the limit of 142 C, and contact is established; 
immediately, the water nearest boils violently, and a kind 
of explosion occurs. Glass-blowers utilize this fact ; they 
plunge the mass of incandescent glass held at the end of 
their rod into water, and, turning it rapidly on itself, they 
fashion it ; afterwards, blowing through their rod, they form 
in the midst of the pasty mass a bubble into which they 
introduce a little water, and then close the opening with 
their finger ; the vapour of this water presses the sides of the 
bubble, swells it, and gradually augments its capacity. All 
this is done without explosion, because the glass is very hot, 
and the water which seems to touch it is in the spheroidal 
state, and evaporates very slowly. 

The singular effects we have just proved must now be 
explained. The experiment has taught us that there is no 
actual contact between the liquid and the red-hot solid, and 
that the temperature of the liquid is always below that at 
which it would boil. It is understood that the vapour pro- 
duced at the surface of the liquid fills the space which 
separates it from the solid ; but the question arises whether 
the elastic force of the vapour is sufficient to maintain the 
separation, and whether the rounded forms of the liquid 
globules result from their evaporation? It seems more natural 
to attribute these forms and the absence of contact to the 
mutual action of the liquid and the solid. A drop of water 
thrown on to a plane surface covered with lamp-black takes 
the spheroidal shape without wetting the surface, exactly as 
if it had been thrown on to the incandescent surface. Doe? 



2l8 THE PHENOMENA AND LAWS OF HEAT. 

it not seem as if these two effects are due to the same 
cause ? Now, when the molecules of a liquid are free from 
other than their own mutual attractions, they always collect 
into a spheroidal mass. If they are seen to take another 
form, it must be supposed that they are under the influence 
of some other exterior force. For example, if the liquid 
spreads out on a surface, wetting it, it would naturally be 
concluded that there is an attractive force which causes the 
molecules of the liquid and those of the surface to adhere 
together, and which overcomes the mutual attractions of 
the molecules of the liquid. It may also be shown that a 
body is wetted by a liquid when the cohesion of the mole- 
cules of the liquid for each other is less than double their 
cohesion for the solid. It has hence been conceived that 
by heating the solid this latter force has been diminished 
to such an extent that the first becomes predominant ; in 
other words, it is the resultant of these forces which com- 
bines with gravity to give such or such a form to the liquid. 
Certain facts, indeed, even lead one to think that the co- 
hesion of the liquid for the solid might be changed into 
repulsion at a sufficiently elevated temperature, and that the 
vapour itself is repulsed by the incandescent surface : for 
example, nitric acid in the spheroidal state does not attack 
a hot copper plate, which could not be if contact existed. 
Whatever may be the precise explanation, we must see in 
the molecular forces the cause of the non-contact which ex- 
periment has demonstrated in the spheroidal state of liquids. 

It remains to be explained why the liquid cannot attain 
the temperature of ebullition. We shall here find the 
application of the laws of heat with which we are already 
acquainted. 

Heat cannot pass easily from a solid to a liquid by con- 
duction, because the thin layer of vapour conducts very 
badly. The liquid is chiefly heated by radiation ; now, a 
part of the rays is reflected from the surface of the liquid, a 
part traverses it, the remainder only being absorbed. This 
absorbed portion of the rays serves to elevate the tempera- 
ture of the liquid and to vaporise it, but especially the 
latter. A very small portion of heat only is employed to 



GREAT FEAT ENDURED BY MEN. 219 

raise the temperature of the liquid ; vaporisation consumes 
nearly all the heat absorbed, and this itself is only a fraction 
of the heat radiated by the incandescent surface. The 
greater the incandescence, the smaller is this fraction ; be- 
cause the luminous rays have only a slight heating power. 
All is thus easily explained, without the necessity of assum- 
ing the existence of a new force, as some have done. 

The heat employed in the transformation of a liquid into 
vapour is really annihilated as heat; it is converted into 
molecular motion, and we shall cite yet another curious 
example. 

It is related that two Englishmen exposed themselves 
in ovens to a temperature exceeding ioo° C, and that 
they came out safe and sound. There is nothing extra- 
ordinary in this experiment if we remember that the human 
body is a tissue impregnated with water ; that this water 
can come to the surface of the skin by perspiration and 
there evaporate. When the body is in a very hot medium, 
the hea,t is employed interiorly to produce work to prepare 
perspiration ; it can only very slightly raise the tempera- 
ture. At the surface an abundant sweat protects the skin ; 
it is the water which comes from the interior of the body 
and which evaporates sufficiently fast to prevent the tem- 
perature from rising to a very great extent. The whole of 
the heat radiated by the oven to the bodies of our hardy 
experimenters may be said to have been destroyed by per- 
spiration, and we need only be astonished at the audacity 
of their enterprise. 



CHAPTER IX. 

ON THE THREE STATES OF MATTER, AND ON THE ARTI- 
FICIAL METHODS OF PRODUCING COLD. 

i. Liquefaction of gases, and solidification of liquids. 

We have already, in the preceding chapters, seen how 
solid bodies pass into the liquid state, and liquid bodies 
into the gaseous state, by the action of heat, and how 
changes, the inverse of these, are produced by cooling. 
The same substances may be, according to circumstances, 
either solid, liquid, or gaseous ; each of these states corre- 
sponds to a particular arrangement of the molecules, which 
is determined by their mutual actions, and by the quantity 
of sensible heat which they contain : and theoretically, any 
substance should be attainable in all of these three states. 
If we have not attained this result with all substances, it is 
because of the insufficiency of our processes. Thus oxygen 
and nitrogen, which constitute the air, at present are only 
known in the gaseous state ; but analogy leads us to 
expect that they may exist in both the liquid and solid 
state under circumstances to which we have not as yet 
been able to submit them. 

We know that the vapour of water takes the liquid state 
by cooling, and that water becomes ice by cooling. Sub- 
jecting a gas to cold is therefore one means of liquefying 
it, of which we have an example in the liquefaction of 
sulphurous acid gas. Fill a large bladder or india-rubber 
bag with this gas, and adapt to the opening a glass tube, 
surrounded by a mixture of salt and ice (fig. 80) ; with this~ 



LIQUEFACTION OF GASES. 



221 



mixture the temperature of the tube will be lowered about 
2o° C. below zero. By pressing the bag lightly, we shall make 
the gas it contains pass slowly into the cool tube, and its 
temperature will thus be considerably lowered. We shall 
obtain in the tube a limpid liquid very volatile, and of a 
penetrating or/our like that of the gas : this liquid boils at 
io° below zero C. under the ordinary pressure. It is this 




jjMiiinifK 

Fig. 80. — Liquefaction of Sulphurous Acid by Cold. 



liquid that we have already employed in its spheroidal state 
to freeze water in a red-hot crucible. 

There is another method of liquefying gases : it is by 
compression. We have seen in the preceding chapter that 
the elastic force of a vapour cannot pass a certain limit 
of value for a given temperature. Thus the vapour of water 
at ioo° C. cannot have an elastic force greater than that of 
the atmosphere ; at 120 Centigrade, the limit is two atmo- 
spheres, and it rises with the temperature. We have given 
a table showing the maximum elastic forces. Physicists 
have analogous tables for a great number of liquids. For 
example, at io° below zero C, the vapour of sulphurous 
acid has a maximum elastic force of one atmosphere; at 
the ordinary temperature, it is three atmospheres. 

Therefore, if we have some sulphurous acid gas at the 
ordinary temperature in the small end of a bent glass tube, 



222 



THE PHENOMENA AND LAWS OF HEAT. 



as shown in fig. 81, we can fill the long end with a column 
of mercury, so as to compress the gas ; and so soon as this 
column shall have attained a height of about sixty inches, 

it will exert on the gas the greatest 
pressure it is able to bear. If 
we add still more mercury, with 
the idea of augmenting the pres- 
sure, the difference between the 
levels in the two branches of the 
tube will be seen to remain in- 
variably at 60 inches, which indi- 
cates that the maximum elastic 
force had been previously at- 
tained. But as the volume of the 
gas diminishes, a small quantity 
of liquid appears on the surface 
of the mercury, and by the con- 
tinuous addition of mercury in 
the long branch, the gas may be 
made to completely disappear. 
Then there will be seen, at the 
extremity of the small branch, a 
liquid resulting from the con- 
densation of the gas. If at the 
commencement of the experi- 
ment this branch contained about 
a gallon of gas, the liquid, after 
complete condensation, would 
occupy about half a cubic inch. 

If it is wished to condense a 
large quantity of gas by this 
method, a gas-pump must be 
used, which pumps the gas con- 
tained in a reservoir, and delivers 
it into the receiver, where it is to 
be kept. 
Davy and Faraday have liquefied a large number of gases 
by the following process. In a thick glass tube closed at 
one end, and bent in the form of the letter V (fig. 82), 




Fig. 81.— Liquefaction of Sul- 
phurous Acid by pressure. 



PROCESS OF DAVY AND FARADAY. 



223 



substances are placed, which, on being heated, disengage 
the gas that it is desirable to condense. Having put these 
matters at the bottom of the tube, the open extremity is 
hermetically closed by the blow-pipe. This end is then 
placed in the mixture of salt and ice, whilst the other end 
is heated. The heat disengages the gas, and, as it cannot 
escape from the tube, its elastic force gradually increases, 




Fig. 82.— Liquefaction of Gases by the process of Davy and Farady. 



and at last attains its maximum value. After this, a liquid 
begins to appear at the cold end, and increases in bulk so 
long as the gas continues to be disengaged. ■ 

This process was applied on a large scale by Thiloner, 
in 1834, to the liquefaction of carbonic acid gas. He con- 
structed an apparatus (fig. 83) composed essentially of two 
very strong metal reservoirs, and furnished with stop-cocks 
of a peculiar pattern. In one of these reservoirs, called 



224 



THE PHENOMENA AND LAWS OF HEAT. 



the generator, bicarbonate of soda is placed, and afterwards 
a copper tube rilled with sulphuric acid, and open at the 
upper extremity ; the reservoir is then closed, and made to 
oscillate on a horizontal axis. The acid, mixing with the 
bicarbonate, disengages carbonic acid gas, which soon 
attains its maximum elastic force, about fifty atmospheres, 
at the ordinary temperature. A communicating tube is then 




Fig. 83.— Thilorier's Apparatus for liquefying Carbonic Acid. 



fitted between the generator and a second reservoir, called 
the receiver, and communication is opened. The carbonic 
acid gas passes into the receiver, and there condenses, 
because the temperature of the generator is always a little 
higher than that of the receiver, in consequence of the 
chemical action which is there affected. A true distillation 
takes place out of the warmer of the two vessels into the 



PROCESS OF THILORIER. 225 

colder; and this being the case, the condensed liquid is 
very pure. At the end of the operation the receiver is 
closed, and the pipe removed. We shall see shortly to 
what use liquid carbonic acid is applied. 

This operation presents great danger, in consequence of 
the enormous pressure existing in the reservoirs : it may 
become fifteen times greater than that of the steam in our 
locomotives, and, if the reservoirs are not sufficiently strong, 
a terrible explosion might take place. An accident of this 
kind caused the death of one of Thilorier's operatives; 
since which the construction of the apparatus has been so 
carefully studied, that it may be used without fear. Each 
reservoir is formed of three superposed metallic envelopes ; 
the interior is of lead, the middle part of copper, and the 
exterior of bands of wrought iron. This apparatus is 
capable of resisting a pressure of 1,000 atmospheres. 

The solidificaton of liquids is always effected by cooling. 
Theoretically, the liquids which contract on passing into the 
solid state might be solidified by compression; but so 
immense a pressure would be necessary, that this process is 
impracticable. On the contrary, it is very easy to produce 
artificial cold, and we shall now notice the various methods 
used. 

2. Artificial production of cold. — Manufacture of ice. — 
Solid carbonic acid. 

One method of producing artificial cold is by availing 
oneself of nocturnal radiation, the effects of which have been 
explained in Chapter IV. We have already seen how ice is 
made in Bengal by this method, but it does not give the 
command of a very intense cold, and we only recall it 
because its simplicity endows it with a certain measure of 
industrial importance. 

Another method results from the relation which we have 
found to exist between heat and mechanical work. When- 
ever a mechanical work is produced without a correspond- 
ing expenditure of work, which arises from the action of a 
motive force, we may observe in the body in which this work 

p 



22b 



THE PHENOMENA AND LAWS OF HEAT. 



is effected, a deficit of sensible heat ; this heat disappears, 
and cannot be found in the neighbouring bodies. It is 
natural to suppose that it has been converted into me- 
chanical work, seeing that it is proportional to the work 
produced. The following is a very simple example. 

A metallic reservoir (fig. 84), closed by a stop-cock, is 
filled with compressed air. A jet of air from it is directed 
on to the bulb of a thermometer, and this indicates fallen 
temperature. If the reservoir were surrounded with water, 
we should also observe a fall in the temperature of the 




Fig. 84. — Cold produced by a Jet of Air. 



water, which proves that a part of the sensible heat of the 
air has disappeared during the exit of the air. This loss 
has afterwards been partially repaired by the heat which the 
surrounding bodies furnish to the cold air remaining in the 
reservoir; but finally both these bodies and the escaped 
air are minus a certain quantity of heat, of which no portion 
can be found. 

Let us now examine the above phenomemon. At the 
moment of its escape from the reservoir, the elastic force of 
the compressed air has lifted the atmospheric air existing 
before the orifice, and the effect produced is mechanically 
the same as if the stop-cock were surmounted by a long 
pipe containing a piston one square decimetre in section, 
and of the weight of 103 kilogrammes, since the weight of 



MANUFACTURE OF ICE. 227 

?uch a piston represents the pressure of the atmosphere. 
Suppose the compressed air raises this piston 4 metres, and 
that then its elastic force has become sufficiently reduced 
to re-establish the equilibrium ; the work produced, in this 
case, will be equivalent to 412 kilogrammetres, and the 
thermal unit will have disappeared. The volume of the gas 
has been augmented to the extent of 40 litres during the 
same time. Whenever a gas extends itself in the atmosphere 
in issuing from a reservoir where it is in a compressed state, 
each augmentation in volume of 40 litres corresponds to a 
work produced, and a quantity of heat lost, as stated above. 

The method of producing cold by the relaxation of gases 
has been applied to the manufacture of ice. Conceive a 
machine in which a piston put in motion by a motive force 
draws atmospheric air into a cylinder, and afterwards com- 
presses it slowly in a reservoir. A certain quantity of 
mechanical work will be expended in this operation, but 
the temperature of the compressed gas will not rise if the 
operation is sufficiently slow to allow the heat created by 
this expenditure of work to pass away into the surrounding 
bodies. This heat disseminated in these bodies is not 
utilizable, and in practice its effects may be neglected. The 
compressed air afterwards expands rapidly in a cylinder 
surrounded by water, and its temperature spontaneously falls 
below zero. It then takes heat from the water ; and that 
again in its turn cools ultimately to zero ; after this it can 
evidently yield no more heat to the gas except by freezing. 
The working of the machine makes these operations con- 
tinuous, and they may be summarized as follow : — 

First period, — Ordinary air is slowly compressed without 
change of temperature ; this necessitates an expenditure of 
work. 

Second period. — The compressed air expands rapidly and 
its temperature falls below zero. 

Third period. — The cold and relaxed air again returns to 
zero and to the ordinary pressure, by taking heat from the 
water which freezes. 

On this principle an industrial process has recently been 
founded in England for the manufacture of ice on a large 



228 



THE PHENOMENA AND LAWS OF HEAT. 



scale. A steam-engine works the air-pump, and, according" 
to Kirk, the inventor, a quantity of ice may be produced 
equal in weight to that of the fuel consumed. 

The relaxation of moist air 
is a cause of cold which acts 
by bringing about the condensa- 
tion of the water vapour it con- 
tains, and even its congelation. 
It is very easy to show experi- 
mentally the formation of a mist 
owing to the action of this cause. 
It is sufficient to put two glass 
reservoirs in communication, the 
one containing air saturated with 
moisture, the other empty (fig. 
85). When the stop-cocks are 
opened a little cloud may be seen 
to appear in the first reservoir 
simultaneously with a whistling 
sound which the air makes in the 
act of rushing into the empty 
reservoir. The mist becomes 
very apparent when placed be- 
tween the eye and the light ; it 
appears dim and often surrounded 
by an iridescent halo. 

When any local circumstances 
determine a diminution of pres- 
sure at any given point of the 
atmosphere, the surrounding air 
extends and occupies the rarefied 
spaces : the augmentation of its 
volume is what we have called 
relaxation, or letting go from the 
state of compression in which it 
had been held. Such is a cause of fog, rain, and even of 
snow, which we must now add to the other examples that 
we have already met with in studying the radiation and con- 
vection of heat, It is quite evident that the motions of the 




Fig. 85. — Condensation of water 
vapour by rarefaction of the air. 



COOLING BY EVAPORATION. 229 

atmosphere due to this cause produce local winds, and that 
it plays a very important part in meteorological phenomena. 

The evaporation of liquids furnishes us with a third 
method for the artificial production of cold, and it is more 
easily applied than the two preceding ones. It has also 
been employed by physicists and chemists in the liquefaction 
of a large number of gases and the solidification of their 
liquids. Alcarazas are vessels of porous earth in which 
water can be kept fresh. They have long been used in 
' Asia, and have been imported by the Arabs into Spain, 
whence they have passed into France. The water contained 
in these vessels sweats through the sides, and, reaching the 
exterior surface, evaporates, by consuming some of the 
sensible heat of the remaining liquid water. The latter 
may thus fall to the temperature of io° C. or 50 F., when 
the exterior temperature is 30 C. or 90 F. The alcaraza 
must be placed in a gentle current of air, so that the air in 
contact with the vase may not be allowed to remain and 
become saturated with moisture. In Bengal, moistened 
leaves are suspended before the windows ; the exterior air 
being very hot and dry enters the rooms by traversing these 
leaves ; it evaporates the water rapidly and cools sufficiently 
to acquire a certain degree of freshness. Here, too, we see 
one of the causes of the coolness of the woods during 
summer. 

During evaporation water may attain the temperature of 
zero, after which, if evaporation continues, congelation takes 
place. The heat produced by the part solidified is then 
consumed by the part vaporised. Each gramme of vapour 
produced absorbs the heat disengaged by eight grammes of 
water in freezing. But to produce ice it is necessary to 
arrange that the water only shall give out heat, and to do 
this the neighbouring substances which are at the ordinary 
temperature must be hindered from giving out any con- 
siderable quantity. This has been realized by Leslie in a 
celebrated experiment. 

A thin capsule of copper, large but shallow, containing a 
little water, is sustained by three metallic supports over a 
vessel filled with concentrated sulphuric acid (fig. 86). This 



23O THE PHENOMENA AND LAWS OF HEAT. 

apparatus is placed under the receiver of an air-pump, by 
which the air is then removed. The water evaporates in- 
stantaneously in the vacuum ; but the evaporation would 
.stop if the vapour formed remained as such in the bell-glass. 
To prevent this, sulphuric acid is used, which absorbs the 




Fig. 86.— Freezing Water by Evaporation. 

vapour as fast as it is produced, and in this manner 
maintains the vacuum. By this ingenious contrivance the 
evaporation of the water is rendered very rapid, and the 
surrounding bodies have not time to furnish the necessary 
heat, so that it is not long before the water remaining in the 
capsule becomes congealed. 

This method has also been applied in England to the 
industrial manufacture of ice ; only, instead of making the 
vacuum by an air-pump, Messrs. Taylor and Martineau fill 
a large reservoir with hot steam, the sides of which are 
afterwards cooled, and the vapour being thus condensed a 
vacuum is formed. Afterwards they make this reservoir 
communicate with the vessel containing the water to be 
frozen, and they absorb the water vapour with sulphuric acid, 
as in Leslie's experiment. 



COOLING BY EVAPORATION. 23 1 

The evaporation of a more volatile liquid than water 
produces a much more intense cold. It is, for instance, very 
easy to freeze water by the evaporation of ether. Water is 
put in a glass tube, and after being surrounded with cotton 
moistened with ether, it is placed in a glass, and the mouth- 




Fig. 87. — Freezing Water by the Evaporation of Ether. 



piece of a bellows is introduced, which is vigorously blown 
(fig. 87). The current of air thus continuously passing over 
the cotton acts on a very large surface of ether, which thus 
evaporates sufficiently fast to freeze the water in the tube. 

If we replace the water by mercury, and the ether by 
liquid sulphurous acid, the mercury may be frozen, which 
indicates a fall in temperature of at least 40 below zero C. 



232 



THE PHENOMENA AND LAWS OF HEAT. 



But this disposition is not to be recommended, in conse- 
quence of the insupportable odour of the sulphurous acid ; 
for this reason other methods of making this experiment 
have been invented which we need not describe here. We 
shall limit ourselves to two more important examples of 
cold produced by evaporation. 

The first is furnished to us by a French manufacture 
already much in use and due to Carre. It is applicable to 




Fig. 88. — Carre's Freezing Apparatus. 



the manufacture of ice, but also to other industrial purposes. 
For example, it is used with advantage for making saline 
solutions, such as sea-water, crystallize. It is known that 
sea-water, after having deposited common salt by evapora- 
tion, still contains other salts dissolved. 

Carre's apparatus is composed essentially of two metal 
reservoirs united by a pipe, and constituting a space perfectly 
closed (fig. 88). One of the reservoirs being filled with an 
aqueous solution of ammonia is heated, whilst the other 
reservoir is plunged into cold water. The ammonia gas is 
disengaged from the solution and condenses in the cold re- 
servoir, exactly as in Faraday's experiment described above. 



SOLIDIFICATION OF CARBONIC ACID. 233 

When all the gas has been condensed, pure water remains 
in the hot reservoir ; it is then removed from the fire and is 
plunged into cold water whilst the reservoir containing the 
ammonia is exposed to the air. The liquid ammonia im- 
mediately begins to evaporate, fills the apparatus, and dis- 
solves in the water as fast as it reaches it; its extreme 
solubility in cold water determines a continuously renewed 
vacuum in the apparatus, and in consequence a very active 
evaporation of the liquid ammonia. The reservoir which 
contains this liquid is therefore considerably cooled, and, as 
it surrounds the cylinder filled with water, the latter is not 
long in freezing. This cylinder is shown separately in fig. 88. 

Our second example is that of the solidification of liquid 
carbonic acid by its own evaporation. We have already 
noticed Thiloriefs apparatus for the liquefaction of carbonic 
acid gas (fig. 83). The liquid is kept in the receiver, at the 
ordinary temperature, under the pressure of 50 atmospheres. 
To get it out, the receiver is turned in such a manner that 
the liquid enters the stop-cock, which is opened with care 
over a vessel. A jet issues forcibly, of which a part evapo- 
rates, its temperature thus becoming lowered to 70 below 
zero C. The liquid contained in the vessel keeps this 
temperature whilst continuing to evaporate ; a part also 
solidifies. 

The phenomenon is still more remarkable when the jet 
of carbonic acid is directed into the air. It cools sufficiently 
for a part to solidify and form white flakes like snow. This 
is a white substance like ice in powder, and is the same 
matter as the gas formed by the combustion of charcoal 
and air. It bears to carbonic acid gas the same relation 
that ice does to the invisible water vapour in the air. If 
this snow-like substance were heated in a suitable manner 
in a partly closed vessel, it would be seen to melt, and the 
liquid produced might boil and be transformed into a gas. 

The solidification of carbonic acid is an example of the 
triumph of mind over matter. After having discovered that 
water is liquefied steam or vapour, and that ice is solidified 
water, man has inquired if there were other substances 
capable of existing in the same three states, and of these he 



234 THE PHENOMENA AND LAWS OF HEAT. 

has found a great number. He has learnt how to melt and 
volatilize the metals ; and later, continually extending his 
knowledge, he has sought to resolve the same question with 
every new substance. Hence it has been considered that 
gases — those subtle matters which are both invisible and 
inappreciable to the touch, but which manifest themselves 
by their weight, their elastic force — are really the vapours 
of certain liquids, and that these liquids might be obtained 
artificially, although Nature never presents them to our sight. 
Man has thus become the creator of a number of bodies 
which he would probably never have met with, but the possi- 
bility of whose existence has been anticipated by him. His 
mind grasps daily new forces of nature, and passes the limits 
which seemed to have been assigned to him by the Supreme 
Creator ; and each of these specific conquests of his intelli- 
gence brings him nearer his Divine Author. 

Van Marum was the first to liquefy gas : this gas was 
ammonia, which is now employed in Carre's apparatus. Since 
1823 Faraday has liquefied and solidified a great number of 
other gases, and as the processes become more perfect the 
number of those which resist this change in their state be- 
comes diminished. There are now only five that have not 
been liquefied, and for this reason they are called perma- 
nent gases : these are oxygen, nitrogen, hydrogen, cabonic 
oxide, and binoxide of nitrogen. The first two constitute 
atmospheric air ; so that we as yet only know the latter in 
the gaseous state : but it is highly probable that in time 
these constituents of the air will be reduced to the liquid 
state by combining great pressures with excessive cold. 

A fourth method for the production of cold is founded 
on the fusion of solid bodies. We know in fact that every 
solid in passing to the liquid state consumes heat. Conse- 
quently, if this change of state is effected under the action 
of other forces than that of the heat of a fire, a part of the 
sensible heat of the surrounding bodies is destroyed, and 
their temperature falls. Fusion may be determined by the 
mutual action of the fusible body and of certain substances 
with which it may be mixed. By associating such substances 
freezing mixtures are formed. 



FREEZING MIXTURES. 235 

The most simple consists of powdered ice and common 
salt mixed in equal quantities. The temperature falls about 
20° C. We have more than once had to make use of this 
method of producing artificial cold. 

How is the ice made to melt ? The salt is soluble in 
water: there is therefore between these two substances an 
attractive action which pre-induces them to constitute the 
solution. The molecules of ice separate from one another, 
and this separation is a mechanical work produced, which 
causes the destruction of a proportional quantity of sensible 
heat. This heat is at first taken from the mixture, afterwards 
from the vessel, the air, and neighbouring bodies : their 
temperature therefore falls. A deposit of ice may be often 
seen on the outside of the vessel; it is formed by the moisture 
from the air being cooled, afterwards condensed into liquid 
drops and finally frozen. The mixture also often appears 
to fume like hot water in consequence of the moisture from 
the air being precipitated in the form of mist. This mist 
does not, therefore, originate in the same manner as that of 
hot water, because the latter is produced by the steam from 
the water, as we have explained in our eighth chapter. 

So far we have only considered the separation of the 
molecules of ice ; but the molecules of salt act in a similar 
manner. We all know that a grain of salt disappears com- 
pletely in a glass of water ; that is to say, it loses its solid 
state by solution and by dissemination throughout the whole 
bulk of the water. Does not this constitute a kind of fusion 
which also consumes heat ? If so, we should obtain cold 
by simply dissolving salt in water. Experiment exactly 
fulfils this prediction. Immerse a very sensible thermometer 
in water, dissolve some salt in it, and a slight fall in temper- 
ature may be observed. 

There are many substances more soluble than common 
salt which produce considerable cold. Nitrate of ammonia, 
a white substance in the form of long fibrous lamina, pro- 
duces a temperature below zero when dissolved in ordinary 
water. If, therefore, we surround a vessel containing pure 
water with such a mixture, the water may be frozen, sup- 
posing the proportions suitable. Such is the principle of 



236 



THE PHENOMENA AND LAWS OF HEAT. 



the family ice-machine represented in section in fig. 89. 
It consists of a metallic vessel which receives the freezing 
mixture, and of a vessel formed of two cones which holds 
water to be frozen in the space between them, so that the 
freezing mixture comes in contact with both the interior 




Fig. 89. — Family Ice-Machine. 



and the exterior of the vessel. A hollow cone of ice is 
thus obtained, which may be easily detached by reversing 
the vessel. 

The most energetic freezing mixture known is composed 
of frozen carbonic acid and ether. With this the temper- 
ature maybe lowered ioo° below zero C. Physicists and 
chemists have utilized this mixture to liquefy and solidify 
many substances for which other means were insufficient. 
We have seen how solid carbonic acid is produced by 
Thilorier's apparatus. When the object is to collect it, the 
issuing jet is directed into a brass box formed of two hemi- 
spherical parts which may be easily separated, and which 
are held by hollow handles (fig. 90, m m). The gas enters the 
box by an opening made at a tangent {d) y and strikes against a 



PROPERTIES OF SOLID CARBONIC ACID. 237 

little blade which it causes to whirl in the box ; a part of the 
gas remains there in the state of solid flakes, the remainder 
escapes by the handles. When the box is full, it is opened 
and the flakes collected. 

Many curious experiments may be made with solid car- 
bonic acid. We have already noticed its spheroidal state 
and its power of freezing mercury in a red-hot crucible. 
Some of its other properties are as follow. 

Placed on a polished surface, it will fly from the hand 
when brought near, because the warmth of the hand rapidly 
vaporises the part nearest to it, and the gas thus escaping 




Fig. 90. — Box for collecting solid Carbonic Acid. 

almost as a jet from the surface of the fragment causes it 
to recoil. 

It may be lightly touched without any sensation being 
felt, because contact does not take place ; the spheroidal 
state is at once induced. Neither is there any danger in 
taking a piece of carbonic acid into the mouth ; a cushion 
envelopes it and prevents it touching the skin. But, as this 
vapour cannot be breathed, care must be taken to hold 
the breath when this experiment is made. The flame of a 
taper may be extinguished by blowing it very gently ; the 
flame is hardly agitated, and goes out quietly because the 
breath removes the air which is necessary to its existence, 
and replaces it by carbonic acid gas. 

Few phenomena appear more prodigious than this : we 
may hold in our mouth, without any harm, a body whose 
temperature is 70 below zero C, and the vapour of which 
poisons the breath. One must have great confidence in 
science to try such an experiment, as in venturing to pass 



238 THE PHENOMENA AND LAWS OF HEAT. 

the hand through a stream of molten metal. But the man 
who loves and seeks the truth, is endowed with a moral force 
which enfranchises him from vulgar fears ; what may appear 
to others to be odd and foolhardy, is to him the striking 
demonstration of a natural law ; he acts with calmness and 
reflection under the influence of a profound admiration. 

If a piece of this solid carbonic acid is touched with the 
hand with sufficient pressure to insure actual contact, the 
hand is burned as if it had touched red-hot iron, and the 
skin is blackened, proving that the organic tissues suffer a 
complete disorganization by excessive cold and heat, and 
that there is little difference between the two cases. 

The following experiments illustrate the intense cold of a 
mixture of solid carbonic acid and ether. A fragment of 
the acid is deposited on the bottom of a reversed glass, and 
ether added little by little ; a cracking is immediately heard, 
the glass is broken in consequence of the unequal con- 
traction of its various parts by rapid cooling. 

By putting some of the same mixture in contact with 
mercury, the latter is instantaneously congealed. With the 
proper proportions, several kilogrammes of solid mercury 
may be prepared in as many minutes, which may be ham- 
mered, cut, or worked, provided it is not touched very 
strongly, for it would burn the fingers. If moulds are filled 
with liquid mercury, and then surrounded by the mixture, 
busts or statuettes of solid mercury may be obtained, which 
look like silver. There temperature is so low, that they 
remain for some time in a solid state ; fusion takes place 
very slowly on their surfaces as soon as they are removed 
from the mixture. 

A very curious experiment is to plunge a piece of solid 
mercury suspended by a thread into ordinary water. The 
mercury melts, and dissolves itself into an infinity of liquid 
threads, which, as they fall, freeze the water in their passage ; 
each thread thus surrounds itself with a tube of ice, through 
which the mercury flows, so long as the fusion continues. 



AGGREGATION AND DISAGGREGATION. 239 

3 . Solution. — Crysta llizaiion. 

We have noticed the principal circumstances in which 
cold is produced, and we now know of the methods at 
command to induce a given substance to take the solid, 
liquid, or gaseous state. We have also met with a state ot 
solution in treating of freezing mixtures. We shall finish 
this chapter with some observations on this state. 

The complete study of the solubility of solids in liquids 
belongs rather to chemistry than to physics; but there is a 
general physical principle verified in this phenomenon, and 
therefore we now speak of it. Numerous examples have 
taught us that the separation of the molecules of a body is 
a mechanical operation which consumes heat, and that, 
inversely, their reunion creates heat. The separation may 
be effected under various circumstances ; it is called, gene- 
rally, disaggregation ; inversely, the reunion is called aggre- 
gation. In a solution, the dissolved solid is disaggregated, 
but the disaggregation which it has suffered must not be 
confounded with fusion properly so called, which we have 
studied in Chapter VII.; because not only are the mole- 
cules of the solid body separated from one another, but 
further, they are associated with those of the dissolving 
liquid, and this second operation is inverse to the first. 
There are also many instances of solution which are effected 
without the disengagement of heat. As a rule, there is on 
one side consumption of heat in the disaggregation of the 
solid body, on the other side a creation of heat in the com- 
bination with the dissolving liquid. The observable effect is 
due to the difference of these two quantities of heat. If the 
first is larger than the second, there is a fall in temperature, 
and a rise if it is smaller. Experiment only would tell us 
which of these two effects will be produced with a given 
solid and liquid. Common sugar produces cold wh^n dis- 
solved in water, which may be rendered evident by a very 
sensible thermometer. When we prepare a glass of eau 
sucree, we therefore cool our beverage. We must conclude 
from this fact that it is the effect of the disaggregation of 
sugar which predominates. 



240 THE PHENOMENA AND LAWS OF HEAT. 

Reciprocally, when a dissolved substance retakes the 
solid state in the midst of the solution, there is an aggrega- 
tion and creation of heat, which might possibly be rendered 
manifest in certain cases in which the separation of the 
solid and the solvent consumes a quantity of heat smaller 
than that created. In fact, such an effect is obtained in 
the following experiment : — 

A solution of sulphate of soda saturated at 32 C. may 
be cooled without depositing salt, if this be done without 
agitation. If a small crystal of sulphate of soda be then 
thrown into the solution, a part of the dissolved salt imme- 
diately takes the solid state. Its molecules separate from 
the water, and aggregate around the little crystal ; fresh 
crystals continue to form on the surface thus presented, 
which soon take the form of long needles with plain faces, 
which ultimately fill the vessel. The beauty of this crys- 
tallization so struck Glauber, a celebrated chemist, who first 
obtained it, that he called this salt set admirable. In 
Chapter VII. we have witnessed the dissection of a block of 
ice by a ray of heat, and we know that by fusion the molecules 
separate from one another in a marvellous order ; we here 
witness the inverse phenomenon ; that is to say, the regular 
reconstruction of a solid structure, the materials of w r hich 
are disseminated in the solution, a harmonious work which 
also reveals to our eyes the existence of molecular forces. 

If a thermometer be placed in the midst of the saline 
solution during crystallization, we shall see it indicate a 
disengagement of heat ; this heat is equal to the work ex- 
pended by the molecular forces, and serves to measure it. 
Every thermal unit represents a work of about 425 kilo- 
grammetres ; or, to be more precise, the molecules of the 
salt, in aggregating to form crystals, have disengaged one 
thermal unit, while their expenditure of work has been equal 
to a weight of 425,000 kilogrammes falling on the earth from 
a height of one millimetre. By taking as a comparison a 
very great weight falling from a small height, we come near 
the proportions which exist in the work of crystallization 
between the amount of the molecular forces, and the dis- 
tances that the molecules have to travel. 



SOLUTION AND CRYSTALLIZATION. 



241 



Solutions crystallize under many other circumstances : we 
have chosen the above example because it allows us to 
recognise the heat disengaged. Solutions usually deposit 
crystals by evaporation or cooling, and when there is no 
agitation, the crystals may grow very large and regular ; 
otherwise they are small, and agglomerated in confused 
masses. Fig. 91 represents some crystals of alum. 

A very pretty experiment in crystallization by evaporation 
consists in depositing a drop of a saline solution on a 




KlG. 91. — Crystallization of Alum. 



microscope slide, which is then observed through a micro- 
scope. The water slowly evaporates, and the molecules of the 
salt being gradually deposited by the liquid collect together, 
and form very regular designs. Every salt has its peculiar 
form of crystallization. The experiment becomes a very beau- 
tiful one when a solar microscope is used, which projects 
on a white screen an image of the saline drop, magnified 
several thousand times. The figures formed on the screen by 

Q 



242 THE PHENOMENA AND LAWS OF HEAT. 

sal-ammoniac resemble the plan of a large town, with its 
streets lined with houses, and crossing each other in every 
direction. 

The geysers of Iceland furnish a good example of natural 
crystallization by solution. We have seen in the preceding 
chapter that they are produced by a water charged with 
silica, a substance which constitutes sand and rock crystal, 
and that this silica, depositing as the water evaporates, gra- 
dually elevates the basin of the geyser. We should find in 
the study of geological phenomena various analogous 
deposits ; for example, those stalactites formed in subter- 
ranean grottoes by calcareous matters present odd appear- 
ances which excite the admiration of travellers. But the 
action of rivers is rather to dissolve than crystallize, because 
the waters are not sufficiently charged with the saline matters 
to deposit them under ordinary circumstances. It is only in 
certain lakes and inland seas that the effects of crystallization 
are met with, as on the shores of the Dead Sea, of which, 
chemical analysis tells us, one fourth of the total weight con- 
sists of salt : it may also be considered as a sea drying itselt 
up. But in the ocean, the proportion of salts dissolved is 
not a thirtieth of the total weight. These salts have been 
calculated to represent a solid bed 16 yards in thickness 
over the whole globe, whilst the water in which they are 
dissolved represents one of 1,100 yards thick. The ocean 
is therefore very far from being saturated ; it is constantly 
dissolving a small part of the solid crust of the globe, and 
it is estimated that the quantity of substances annually dis- 
solved would form a layer y^g-^ of an inch in thickness 
over the whole surface of the globe. This dissolving action 
causes a disappearance of heat too slight, it is true, to give 
rise to remarkable phenomena, but which must be taken 
into account in a study of the globe at large. 



CHAPTER X, 

HEAT UPON THE TERRESTRIAL GLOBE* 

I. Equilibrium of heat on the surface of the globe. — Law 
of the conservation of energy. 

The earth is formed of an excessively hot central mass, 
and of a superficial crust, upon which repose the continents, 
the seas, and the atmosphere. Every consideration leads 
us to believe that the central mass is in a liquid condition, 
while the shell we know is formed of a solid crust, the thick- 
ness of which is barely the one hundred and fiftieth part of 
the total diameter of the earth. An idea maybe formed of 
their relative bulk by imagining a sheet of paper to cover 
a sphere eight inches in diameter; the sheet of paper 
represents the solid crust of the earth, whilst the sphere 
represents the liquid centre. We have shown in Chapter I. 
that at a certain depth in the solid crust the tempera- 
ture is invariable through all the changes of season on the 
surface. Above this region the temperature is subject to 
both regular and accidental variations ; below it the tem- 
perature increases regularly with the depth, at the rate 
of about i° C. in every hundred feet. Such is the result 
of numerous observations extending over a period of two 
hundred years. 

The existence of a region of invariable temperature in 
the solid crust of the earth indicates a state of calorific 
equilibrium, a compensation between the heat lost and the 
heat gained by our globe, or at least an excessive slowness 
in the thermal changes of the molten interior, so that we 



244 THE PHENOMENA AND LAWS OF HEAT. 

can admit the constancy of this temperature when we have 
under consideration a period of several centuries. We de- 
pend on this when we seek the laws of the distribution of 
heat above the superficial crust, whether at various epochs 
or in various places. This distribution has the greatest 
interest for us, since the regulation of living beings depends 
on it ; and we shall now endeavour to trace its principal 
laws. 

The atmosphere, the ocean, and all that part of the 
earth's crust situated above the region of invariable tem- 
perature, are subject to two contrary actions which just 
balance each other ; that is to say, the heating action of the 
sun and the cooling action of the celestial spaces. The 
solar rays which strike against the burning sands of the 
desert are in part reflected, in part absorbed. The absorbed 
heat raises the temperature of the sand ; but so soon as the 
sun sinks below the horizon, the sand in its turn radiating 
towards the celestial spaces restores the heat it had re- 
ceived, and compensation is established. 

But, usually, there is a great number of intermediate opera- 
tions between the arrival of a solar ray on the earth and the 
return towards space of an equivalent terrestrial ray ; these 
operations are accomplished both in inorganic matter and 
in organized beings, and in this manner the solar heat deter- 
mines life and motion on the surface of the globe. When 
the sun is absent, the forces of nature are in equilibrium : 
it is the time of repose. When the sun returns, the equi- 
librium is broken ; a new force excites all the others, and 
a fresh arrangement becomes necessary. The alternation 
of day and night is thus an incessant cause of activity. 

Through all the evolutions of matter one grand law pre- 
vails ; namely, the conservation of energy. The energy or 
power of motion is transmitted from one body to another 
with neither augmentation nor diminution, and the effects 
which it produces differing among themselves by their form, 
are always equivalent. It is only of the effects that we can 
have any exact knowledge. As to their cause, we have only 
the sentiment of its existence, and we are totally impotent 
to assign to it any particular nature. We speak of forces, but 



HISTORY OF A DROP OF WATER. 245 

we do not see them ; they are the mysterious causes of the 
phenomena which we contemplate ; they emanate from the 
Divine power, and it does not fall to our lot to have any 
knowledge of their intimate nature. Our part is to atten- 
tively observe the phenomena, and seek the links which 
connect them. 

If we follow the various operations accomplished by solar 
heat acting upon matter, we shall verify the assertion of the 
conservation of force in the same time that we sum up the 
laws treated of in the preceding chapter. A simple drop of 
water will serve as an example. Leaving the ocean in the 
form of vapour, it traverses the atmospheric regions, and 
falls as snow on the glaciers of high mountains; it after- 
wards redescends as water towards the sea, and in its pas- 
sage it meets with the vegetable and animal existences, which 
cause it to undergo marvellous metamorphoses before it is 
again restored to the ocean. 

We will suppose our drop of water to weigh nine grammes, 
and we will now endeavour to follow it through its suc- 
cessive transformations, according to the data of modern 
science. When heated by the sun in the ocean it is re- 
duced into vapour, and the heat it consumes represents an 
expenditure of energy capable of lifting a weight of one 
kilogramme more than 2,300 metres high. The vapour 
mixed with the air rises into the atmosphere, where it con- 
tributes towards the production of winds and of various 
effects recognised in the science of meteorology. Gifted 
with the faculty of absorbing and emitting radiant heat in 
a greater degree than dry air, the vapour of water moderates, 
as by a screen, the cooling action of the celestial spaces ; 
thanks to its influence, the atmosphere serves as a garment 
to the earth. During its stay in the atmosphere our drop 
of water becomes hot and cold by turns, so that it serves as 
a kind of vehicle to the heat, and a perfect compensation is 
established between the heat it absorbs and that which it 
restores. It soon condenses as a cloud, and then a disen- 
gagement of heat takes place. Suppose that the conden- 
sation is effected at the same temperature as the initial 
vaporisation, there will be perfect equality between the heat 




246 TKE PHENOMENA AND LAWS OFl 

,. , , ., , . , ,ed in its vaporisa- 

disengaged and the heat previously expend nine ram J„ es oi 

turn; in other words, the condensation _of , f one f ilogramme 
vapour is equal to the impact of a weigh to know ^ such 
falling from a height of 2,300 metres. W^ £ capable 

a blow develops a certain quantity of actn . k ^( ps The 
of producing mechanical effects in exter,, of ^ heat 
same effects may be produced by the . ^ th {n ^ 
created by the condensation of this vapoi ^ ater has si j 
transport from the ocean to the cloud, the j t first recei ^ e ^ 
transmitted to the atmosphere the energy transmitted b 
from the sun. This energy is afterward ■ h fill rpWl -.:i 
radiation to the innumerable bodies wr 
space. ^ t j ie atmoS p ner } c 

High mountains condense a large part o diati ^ er in 

moisture. Endowed with considerable ra^ the ° ir F situated 
consequence of the purity and rarefaction c duri the . fa 
above them, they cool the surrounding air eir gi( j° s T ^ es e 
and the clouds become heaped around th _. reso i* e th em . 
clouds also cool by their own radiation, a ^^ rige above " 
selves into snow. The snowy summits ^ tfae condensa 
the eternal ice realize on an immense sea ^.i 0/w ,„ 7 rpi 
tion of the vapours formed on the surface c dktillato v r anna- 
ocean and the mountains constitute a true n . - ^ \ ^ n 
T_- -l .i • ^ r L. teat or jrrd All 

ratus, in which the sun is the source of t n¥n ST7^L. ^c 

. .' c 1 • >x • • ^ ^, e water- vapour ot 

the ice of our glaciers owes its origin to th gun for t £ e ^ 

the ocean ; and the heat furnished by the „, ij „„a:„¥ 4.^ 
, . r \i- • ^ ..I 4. -4. would sumce to 

duction of this vapour is so great that it qp _ n fh f 

melt five times the weight of iron. We h j formation of 
equal quantity of heat is created by the sin y calorific effects 
these clouds. We must now realize the snow 
which accompany their transformation into^' di 

In becoming solid, nine grammes of , faUi a ^\Xt 
heat equal to the blow of one kilogramm< ig dissipated by 
of 300 metres, and that quantity of energy^ the f usion Q f 
radiation. But the glacier melts in time, Werse tion 

nine grammes of ice represents just the ^ extent, the 
of the preceding. To preserve an lnvari^ it ing , the 
glacier must lose as much water by fusion ^ th ° n a *riect 
condensation of atmospheric moisture, an< F 






CONSERVATION OF FORCE. 247 

compensation exists between the heat created and the heat 
expended. The energy is here derived from the sun, which 
determines the partial fusion of the glacier, either directly 
or by the intermediate action of the earth, and it appears 
simply transferred to the earth from the celestial spaces. 

Our drop of water has now again become liquid, but it 
is at a great height above the ocean — say 4000 metres. 
We must add to the effects above described the mechanical 
work which represents the transport of nine grammes to 
such a height. The expenditure of energy in this transport 
would be capable of lifting a kilogramme to the height of 
thirty-six metres, and this also is derived from solar action, 
corresponding to the displacement of the atmospheric strata, 
which we have spoken of under the name of Convection. 
This effect is recompensed by the return of the water of 
the glacier to the ocean. Gravity causes the drops to col- 
lect at the base of the glacier, where they combine and 
form streams and rivers, and ultimately find their way into 
the sea. In the descent from a source situated at a height 
of 4000 metres, nine grammes of water would develop an 
amount of energy equal to the preceding. 

By the simple friction of the bodies which hinder its 
progress, this water is able to return the heat of which it had 
previouly robbed the atmosphere. But the forms under 
which the energy may reappear are varied to infinity: its 
transformations do not stop there. The river waters the 
fertile plains, and meanders through a thousand sinuosities 
in accordance with the inclination of the ground. It would 
almost seem that the earth instinctively sought to retain 
these waters in order to nourish the innumerable beings 
which live upon its surface. The vegetable needs various 
elements to compose its organs; it needs water for the 
circulation and elaboration of these elements ; it needs 
also hydrogen. But to fix in its tissues one gramme of this 
substance, the vegetable must first decompose nine grammes 
of water. The sun furnishes the motive force necessary to 
this work, which is equivalent to the elevation of a weight 
of one kilogramme more than fourteen kilometres high. It 
is under this powerful influence that the plant appropriates 



94% THE PHENOMENA AND LAWS OF HEAT. 

one gramme of hydrogen. The plant will afterwards serve as 
food to some animal, in which will reappear all the energy 
previously expended. Each gramme of hydrogen which is 
submitted in the blood of the animal to the action of respi- 
ration, disengages a quantity of heat capable of elevating 
one kilogramme to the height of fourteen kilometres, by re- 
combining with the oxygen of the air to reconstitute nine 
grammes of water. Such is the admirable balance reigning 
throughout nature, the sublime simplicity of which reveals 
to us the infinite wisdom of an all-powerful Creator. 

To resume: we see that, in the circulation of nine grammes 
of water through the ocean, the atmosphere, the glacier, the 
river, the vegetable, and the animal, a work is produced of 
16,636 kilogrammetres, and an equal work expended. This 
represents thirty-nine thermal units, allowing a thermal unit 
to be equal to 425 kilogrammetres. This heat has been fur- 
nished by the sun, and finally it is transmitted to the celestial 
spaces by the radiation of the earth. Thus as Professor 
Tyndall has so eloquently shown, we are directly indebted 
to the solar rays for all the multiplied powers exhibited on 
the globe. There are so many special forms of the power 
of the sun, so many mechanical arrangements in which that 
great motive force exhibits its method of working in the 
interval, so to speak, between the radiation of heat from its 
source and its return into pure space. 

The time separating the moments which the solar energy 
is expending from that in which an equal energy is put in 
activity, may be considerable, so that the energy may seem 
shut up in certain terrestrial bodies. For example, our 
coal-mines are the remains of immense forests which have 
existed on the earth long before the advent of man. Buried 
beneath the waters by geological changes, they have under- 
gone a slow decomposition, and their carbon has been set 
at liberty. Each kilogramme of carbon comes from the 
carbonic acid which the vegetable productions of these 
forests have decomposed during their life under the influence 
of the sun, and this decomposition has entailed an expendi- 
ture of energy sufficient to raise one kilogramme 3400 kilo- 
metres high. We burn coal, and this energy again comes 



DISTRIBUTION OF TEMPERATURE. 249 

to light ; it has been preserved intact, and we utilize the 
Aeat sent, radiated by the sun to the earth thousands of 
years ago. The utilization is complete when we simply use 
the coal in a stove for heating purposes. But if we wish to 
convert the energy into mechanical work, we know that it 
is impossible in our engines to avoid the disengagement 
and loss of a large proportion of sensible heat, or to convert 
all the heat of the combustion into work. We can scarcely 
raise with one kilogramme of coal burnt under the boiler of 
a steam-engine a weight of one kilogramme to a height of 
135 kilometres; the greater part of the energy is developed 
in the form of heat. In any case, however, the great prin- 
ciple of the conservation of force is verified by experience. 
It is this principle which establishes a correlation between 
all physical phenomena, because the energy communicated 
by matter may be manifested otherwise than by heat and 
mechanical work ; it may, for instance, take the form of 
electricity ; but we have been careful to study phenomena in 
which these forces do not intervene. It is by separating 
the natural forces from one another, making them act sepa- 
rately on matter, that a knowledge of their laws may be 
obtained. When science becomes more advanced, it may 
be attempted to embrace under one formula all the incom- 
plete results now arrived at, and thus prepare a synthesis 
which should summarize all science. 



2. Distribution of temperature on the earth* s surface. — 
Climates. — Effects of at?nospheric moisture. 

Having now acquired a general idea of what is meant by the 
equilibrium of heat on the surface of the globe, we proceed 
to consider the distribution of temperatures. If at every 
point of the earth's surface the amount of heat lost and that 
gained were constantly equal, the temperature would remain 
constant. A thousand causes interfere with this equality, 
and the temperature varies periodically according to certain 
laws, in such a manner that there is always a conservation 
of energy. Living beings constantly require the concur- 
rence of heat in a determinate proportion, and also during 



£$0 THE PHENOMENA AND LAWS OF HEAT. 

a limited time. The evolution of the matter which con- 
stitutes their organs has been neither too slow nor too quick : 
hence certain limits to the temperatures to which such beings 
can be exposed without perishing. Man will suffer varia- 
tions of (180 ) F. of temperature, live at 63 F. below the 
freezing-point, like Captain Black when travelling in North 
America in search of Captain Ross, and at 117 F., the 
maximum temperature observed at Esne, in Egypt. But 
most animals and vegetables have not nearly so wide a 
range. 

The study of temperature on the surface of the earth, 
and the search for the laws which regulate it, belong to a 
branch of physics but recently founded. The study of 
meteorology has received considerable impulse of late years 
from many eminent investigators ; it is by an active centrali- 
zation such as that undertaken by Le Verrier, director of 
the Observatory at Paris, that a conclusion may be drawn 
from the numerous observations daily made in all parts of 
the world, and laws formulated of incontestable utility. We 
shall limit ourselves here to the indication of the principal 
causes of the regular variations in the temperature of the air. 

First, consider what passes in a given inland spot. When 
the sun rises, his oblique rays begin to warm the air and the 
earth. The latter absorbs more heat than the air, a part of 
which it afterwards yields to the lower strata of air by radia- 
tion, conduction, and convection. On the other hand, the 
celestial spaces exert a cooling action on the air and the 
soil, and this is so much the greater as the temperature of 
the water is higher. This action is therefore slight at the 
break of day, and it is the warmth of the sun which initiates 
it. As the sun rises its rays arrive less obliquely and bring 
more heat ; the temperature gradually rises so long as the 
radiation outwards has not become sufficiently great to com- 
pensate for the absorptfon of solar heat. After mid-day, 
when the sun, having attained its greatest elevation, has 
already somewhat descended, and the obliquity of its rays 
augments whilst their ardour diminishes, there is a time when 
the heat received exactly equals the heat lost by radiation. 
The temperature is then at its maximum. From this 



HOW TO OBSERVE TEMPERATURE. 



25 



moment the cooling action decreases less quickly than the 
warming action, and the temperature continues to diminish 
until the sun sets, and notwithstanding the presence of the 
sun above the horizon ; it decreases further during the 
whole night. The minimum of temperature is therefore a 
little before sun-rise; it precedes a short time the actual 
appearance of the sun, because the solar rays reach the 
atmosphere before they reach the earth, and their heat is 
diffused in the air like the light which produces twilight. 

There are certain indispensable precautions to be taken 
in making observations upon the temperature of the air. 




Fig. 92. — Maximum and minimum Thermometers. 



The thermometer must be exposed to the north, in the shade, 
and protected from the glow of neighbouring walls ; the 
air must be allowed to circulate freely around the instru- 
ment ; and before making the observation it should be waved 
for a few minutes in the air to avoid the effects of radiation. 
When hourly observations have been made during a whole 
day, the mean temperature of the day is calculated by adding 
the figures representing the 24 observations and dividing 
their sum by 24. The same number is obtained by halving 
the maximum and minimum temperature of the day ; this 
allows the 24 hourly observations to be substituted by two 
made once in the day, with the help of special instruments 
which mark themselves the maximum and minimum. The 
most simple apparatus consists of two thermometers fixed 
horizontally on the same board (fig. 92). One is a mercury 



25 2 THE PHENOMENA AND LAWS OF HEAT. 

thermometer : it indicates the maximum. The other con- 
tains alcohol, and indicates the minimum. To effect this 
the thread of mercury, when the temperature rises, pushes a 
little iron cylinder forward, which remains in its place when 
the temperature falls. The alcohol of the second ther- 
mometer, on the contrary, contains a little enamel cylinder 
which it drags by adherence when the alcohol contracts, 
leaving it, however, in its place when the alcohol expands. 
The different action in these two thermometers is easily ex- 
plained : the mercury is incapable of wetting either the iron 
or the glass, and is therefore unable to pass round the little 
iron cylinder; whilst the alcohol, wetting both the enamel and 
the glass, completely surrounds the latter cylinder. Suppose 
the cylinder placed at the end of the alcoholic column, and 
the alcohol to contract, it drags the cylinder without ceasing 
to surround it ; if, on the contrary, the alcohol expands, it 
leaves the cylinder stationary, and itself passes along in the 
space between the cylinder and the glass tube. When using 
the apparatus, the end of the tablet on the right is lowered, 
and the little cylinders are caused by gravity to fall to the 
extremities of their respective liquid columns. The tablet 
is replaced horizontally, and twenty-four hours afterwards 
the position of the two cylinders is noted. The little 
cylinders are then again adjusted, and so on. 

The mean daily temperature of a given place varies ac- 
cording to the season. After the winter solstice, the time 
during which the sun remains above the horizon increases 
daily until the summer solstice; and at the same time it 
rises higher and higher in the heavens in its diurnal revo- 
lution. It therefore acts stronger and stronger, until, ulti- 
mately, the earth is heated more during the day than it 
cools at night. At the summer solstice, the solar action 
has reached its maximum ; but the cooling action of the 
celestial spaces does not yet compensate it : compensation 
does not occur for some time after, when the solar action is 
undergoing a decrease in consequence of the diminution in 
the length of the day and the height of the sun. The daily 
mean temperature at this moment attains its maximum. 
Afterwards, until the winter solstice, the earth cools more 



HOW TO OBSERVE TEMPERATURE. 253 

during the night than it is warmed during the day, and the 
temperature falls. 

As a crowd of local or accidental circumstances must be 
added to the above cause, the variations in the mean tem- 
peratures of each day become complicated ; their effect, 
however, disappears by calculating the monthly means of 
several years. To do this the sum of the daily means is 
divided by the number of days in the month, and hence a 
monthly rate of mean temperature is obtained. This cal- 
culation is made with the means obtained each month 
during a large number of years. The following are the 
results calculated by Bouvard for Paris, after a series of 
observations extending over sixteen years. Instead of the 
Centigrade scale used by Bouvard, we give the figures 
according to the scale of Fahrenheit : — 

JAN. FEB. MARCH. APRIL. MAY. JUNE. 

3S°. 6 39° * 44°.6 5 l0 .3 S7°.2 62°.6 

JULY. AUG. SEPT. OCT. NOV. DEC. 

65°. 7 64°.8 6o°.6 5 2 . 7 44°.6 39 

These figures show that the maximum is reached in July 
— that is to say, after the summer solstice. By taking the 
sum of the monthly means given in the above table, and 
dividing it by 12, the mean temperature of Paris is obtained ; 
this is 51°. 4, which is very nearly the mean of the month 
of April. 

This number does not, however, suffice to characterise 
the action of the sun on a given spot. The extremes ot 
temperature exercise the greatest influence on animals and 
vegetables. That a plant may not freeze, or that a fruit may 
ripen, the temperature must be neither too low in the one 
instance nor too high in the other. Further, it is necessary 
in this estimate to consider a large number of observations 
so as to eliminate the influences of accidental causes ; and 
it is by comparing the mean temperature of the warmest 
month with that of the coldest, that the climate of the 
place is determined. The preceding table gives us 6$°.j 
and 35°.6 for Paris ; the difference is 30°.!. This figure 
measures the range of the climate of Paris. Such a climate 



254 THE PHENOMENA AND LAWS OF HEAT. 

is variable, because the absolute temperature may acci- 
dentally rise to ioo° F. or fall to io° below zero. Hence 
exceptional winters and summers that cannot be foreseen in 
the present state of meteorology. 

We shall now consider the causes which influence the 
mean temperature and climate of the various countries. The 
farther we go from the equator towards the poles, the more 
obliquely do the solar rays fall on the surface of the globe. 
In consequence, their action becomes gradually less intense, 
and the mean temperature diminishes. When we reach the 
polar regions, we find the atmospheric vapours are con- 
tinually condensed to snow, and the surface of the seas is 
frozen. Immense fields of ice, annually changing their 
position, put an end to navigation, and render the explora- 
tion of these regions almost impossible. Dr Kane over- 
came these difficulties in his memorable expedition to the 
Arctic Seas • but, in spite of the efforts of Cook, Weddel, 
Dumont d'Urville, and James Ross, we have no certain 
knowledge of the southern arctic regions, where, however, 
we cannot reasonably doubt there is also a free polar sea 
surrounded with icy barriers. 

The configuration of the continents and the marine cur- 
rents cause great differences in the distribution of heat in 
the two hemispheres. In the northern hemisphere we find 
vast continents extending very nearly to the pole ; in the 
southern hemisphere, on the contrary, Africa and America 
taper off to points, and present much smaller superficies 
than that of the seas. The southern extremity of America, 
the part which comes nearest to the South Pole, is not 
farther removed from the equator than Denmark. As the 
sea reflects from the surface a larger proportion of the solar 
rays than the land, the heat absorbed by the southern 
hemisphere should be less than that absorbed by ours. 
This, in fact, we find to be the case at the extreme southern 
point of America — the mean temperature is lower than that 
of Denmark. 

Seas have the effect of softening a climate. The vapour 
of water ceaselessly absorbed by the atmospheric air acts as 
a kind of clothing, which preserves the soil from too great 



VARIETIES OF CLIMATE. 255 

warmth or cold. Moreover, the marine currents distribute 
on the coasts the heat which the whole ocean receives from 
the sun, or carries from them a part of that which the land 
has absorbed to deliver it elsewhere. Thus the mildest 
climates are to be met with among islands where the differ- 
ence between the mean temperature of the hottest month 
and that of the coldest is not more than a few degrees, 
and where moderately hot summers are followed by mild 
winters. Such a climate is called constant ; it is met with 
in South America, where immense tree-ferns grow, and 
where the magnificent vegetation of the tropics extends 
much further from the equator than in the northern hemi- 
sphere. Hence, in the same mean temperature as that of 
France, orchids of many curious varieties and vanilla grow 
freely in those countries, although the same species need 
the protection of our greenhouses. 

In England we find again a very striking example of the 
influence of moisture on the climate and on the mean tem- 
perature, which at London is about 50 F. At Irkoutsk, the 
capital of Siberia, which is at about the same distance from 
the equator as London, the mean temperature is a little 
below 3 2 F. At London the mean temperature of the cold- 
est month is 37°-4 F., and that of the hottest month 64 F. 
At Irkoutsk these temperatures are 3 below zero F., and 
63°.5 F. above. The differences of these extreme tem- 
peratures are thus 2 6°. 6 F. for London, and 6o°.5 F. for 
Irkoutsk. This last town is the type of an excessive 
dimate. 

It is easy to explain this enormous difference between 
two localities which, by their position on the surface of the 
globe with respect to the sun, ought to receive the same 
quantity of heat. We have already mentioned the influ- 
ence of the Gulf-stream on the climate of England. This 
vast marine current carries to her shores the heat absorbed 
in the Gulf of Mexico and in the equatorial regions of the 
Atlantic. The atmospheric currents carry in the same direc- 
tion large quantities of water vapour arising from the evapo- 
ration of the ocean : this vapour, condensing in England, 
disengages as much as had been absorbed in its formation 



256 THE PHENOMENA AND LAWS OF HEAT. 

in the tropics. It is a kind of aerial carrier, which draws 
the heat from its source and carries it to a distance. At 
London the south-west wind blows for nine months in the 
year, and whenever it brings rain it brings additional heat : 
hence the mild temperature of the climate. Intense colds 
and excessive heats are alike prevented by the moistness of 
the atmosphere, which moderates in the summer the ardour 
of the sun by absorbing its rays, and in winter preserves 
the terrestrial heat by opposing its radiation towards the 
celestial spaces. According to Tyndalfs experiments, the 
water vapour contained in the atmosphere would exercise 
on radiant heat an absorption seventy-two times greater 
than that of dry air. England, in fact, may be said to 
enjoy the sheltering advantage of a heaven-built screen-; 
and, if such plants as the vine do not flourish in the island 
for want of a sufficient heat in the summer, others are able to 
live there which could not support the cold of continental 
winters. In the north-east of Ireland, according to Hum- 
boldt, the myrtle grows with as much strength as in 
Portugal ; it hardly freezes there in winter, but the summei 
heat is not sufficient to ripen the grape. On the coasts 
of Devonshire, camellias pass the winter unsheltered in 
the open ground, and espalier orange-trees may be seen 
bearing fruit* 

The circumstances are very different at Irkoutsk. At a 
great distance from the sea the air there is very dry, and no 
longer preserves the soil. The winds bring only masses of 
dry air frozen by the polar regions or heated by the deserts 
of Central Asia. In winter, Lake Baikal, near Irkoutsk, is 
frozen during a long time, and may be passed in a sledge 
during all the period from January to April inclusive. Some 
very curious details of this country are given by a traveller 
who visited it in the month of January. The lake — 100 
leagues long, 15 broad — resembled a petrified sea through- 
out its whole extent. " Along the coasts might be seen on 
the sides of the rocks and in the ravines immense icicles, 
which looked like frozen cascades ; these were caused by 
the foam of the lake, which, breaking on the shores during 

* Becquerel's " Elements of Terrestrial Physics and Meteorology." 



CHANGES OF CLIMATE, «57 

a tempest, had been frozen before it could fall back." 
Having started a sledge on this frozen plain, it seemed to 
our traveller as if he were starting in a post-chaise from 
Dieppe for America. Under the ice, two or three yards in 
thickness, strange noises were heard, and occasionally an 
agitation was felt as if the ice and water were not in 
contact. The captive waters lifted themselves from the 
depths, and rushed with fury against the icy vault which 
covered them. In summer, on the contrary, the tempera- 
ture at Irkoutsk is maintained often at 8o° F. for several 
weeks, and the lake is navigated by steamers. The south 
winds would cause much greater summer heat if they were 
not stopped by the Altai Mountains, which shelter the 
country from the wind of the desert. 

The elevation of a spot is one cause of cold. The air 
becoming less dense as we rise absorbs less heat, and its 
temperature decreases. In fact, this is what results from 
the observations made, whether on mountains or in balloon 
ascents. Thus, at the Great St. Bernard the mean tem- 
perature is 3 3 . 8 F. The law of this variation depends on 
a number of circumstances; and we must not now enter 
into any details on this question, nor on that of the height 
of the perpetual snows, which forms part of it. 

The study we have made of the general causes which 
determine the mean temperature and the climate of differ- 
ent parts of the globe, suffices to show on what principles 
their explanation rests. We shall finish this book by 
considering the modifications effected by time on the dis- 
tribution of terrestrial heat within the historical period, and 
those which may possibly be brought about in future ages. 

3. The changes which have occurred in the distribution of 
Heat previous to our epoch* — Geological revolutions. 

In Palestine, in the time of Moses, the date and raisin 
ripened ; from which facts we may calculate approximately 
the mean temperature of that country at the time. In fact, 
the date does not now ripen at Catania, the mean tempera- 
ture of which is 64°.4 F. A tempeiature of 70 F. is neces- 



£$8 THE PHENOMENA AND LAWS OF HEAT. 

sary to ripen it, as at Algiers. The mean temperature could 
not, therefore, have been less than 70 F. in Palestine. On 
the other hand, the vine is not cultivated in hot countries 
where the mean temperature is more than 71 6° F. In 
Persia, 73°-4 represents the highest limit ; and there they 
are obliged to shelter the vines from the heat of the sun. 
The mean temperature of Palestine could not, therefore, have 
been above 7i°.6 in Biblical times. In accepting 7o°.7 F. 
we cannot be far from the truth. At the present day, 
observations made in Jerusalem give a little higher than 
70 F. Therefore, for more than 3,000 years the climate 
of Palestine has not been modified to an appreciable 
extent. In France the climate seems to have suffered 
some change. Formerly the vine was cultivated in the 
Vivarais to a height of 2,000 feet ; now the grape no longer 
ripens there. Every one knows how the Suresnes wine 
has degenerated since the time when it was served at the 
table of the Emperor Julian ; and we have another similar 
example in the wines of Beauvais and Etampes, which were 
esteemed in the time of Philip Augustus. 

In England the vine was also cultivated in certain locali- 
ties where now it is necessary to shelter it from the cold 
winds. Such modifications have been brought about by the 
cultivation of old soil, as fallow land, the reclamation of 
forests, and the drying up of ponds and marshes. An 
abundant vegetation, such as that of old forest lands, has 
the effect of condensing atmospheric moisture, and we 
know that this condensation is accompanied by a disen- 
gagement of heat. In a wooded country, streams are abun- 
dant ; their waters gather together in the rivers and ponds, 
and they maintain a beneficent moisture in the atmosphere. 
Forests, too, serve as shelter from the winds and diminish 
their violence. When these natural features are changed 
by the hand of man, the climate loses its mildness ; the 
mean temperature tends to become lower, the winters being 
at the same time colder, and the summers hotter. Now, 
such plants as the vine especially require a mild climate ; 
to ripen the grapes the summer's heat must be prolonged 
into the autumn, and a change of season must not be 



COOLING OF THE EARTH. 259 

sudden : atmospheric moisture acts in these respects as an 
indispensable regulator. 

From these observations we should deduce a very impor- 
tant consequence relative to the influence of the solar 
action and of the earth's own central heat. Their variation 
is altogether inappreciable within the historical period ; and 
yet should we not imagine that terrestrial heat would be 
slowly dissipated by radiation ? According to Saussure and 
Fourier, the heat lost by the earth in one century is capable 
of melting a bed of ice over the whole earth about ten feet 
in thickness. It is one thousand times less than the heat 
sent from the sun to the earth in the same time. According 
to this, the temperature of the earth would fall about 2° F. 
in 57,600 centuries. It is quite impossible to calculate at 
what period our globe was a mass of incandescent liquid, 
the solidification of which commenced on the surface. 

The history of the changes which have taken place on 
the earth in these incommensurable times is written in the 
numerous strata of the earth, which are interposed, like the 
leaves of a book, in a determined order. Geology is the 
science which, teaching us to read in this gigantic book, has 
made us acquainted with the animals and the vegetables 
which have successively lived on its continents and in its 
seas, the revolutions which have made the mountains rise, 
and which have at various epochs changed the aspect 
of our planet. It shows us that if the cooling of the 
central part does not sensibly influence the mean tempera- 
ture of the atmosphere, it produces a slow contraction 
which at long intervals of time brings about changes in the 
solid crust. 

By admitting the figure which we have given for the 
cooling of the earth, and by supposing that the solid 
crust contracts like glass, we find that the earth's radius, 
which is about 4,000 miles, diminishes about the four- 
thousandth part of an inch annually. The diminution in 
bulk which would result from a contraction five times less 
would be one cubic kilometre. 

Imagine a hollow sphere filled with liquid and gas, and 
presenting several fissures. If the sphere contracts, the 



l6o THE PHENOMENA AND LAWS OF HEAT. 

interior pressure will augment, and the fluid it contains will 
come out by the fissures. 

Such is a rough image of what passes at the present time. 
Three hundred volcanoes exist on the earth's surface, im- 
mense fissures, from which flows incandescent lava, while 
mixtures of gas-cinders and vitrified rocks are cast out by 
the fire. The whole mass thrown out in one year measures 
one cubic kilometre. This result confirms those at which 
we arrive by other considerations. 

But the effect of terrestrial contraction does not stop 
here. The fluid centre, in contracting, leaves an empty 
space between its surface and that of the solid crust. It 
may, therefore, be in a state of continual fluctuation, submit 
like the ocean to the attraction of the moon, have its tides, 
its waves, and its tempests. Every imaginable chemical 
operation might be effected on the surface of this sea of 
fire ; hence the subterranean detonations, the oscillations 
of the solid crust, and the trembling of the earth, most 
frequent in the equatorial regions, where the speed of the 
diurnal rotation is greatest, and the lunar action is most 
strongly felt. In fine, the unequal thickness of the crust at 
different points occasions an unequal collapse, and immense 
fractures take place along the lines of least resistance. At 
such times a terrible overthrow of the established order 
of things takes place on the surface of the earth. Wherever 
there is a fracture a part of the crust sinks in, whilst the 
neighbouring part rises; a chain of mountains may be 
created in a moment, and the volcanoes vomit their in- 
flammable matters wherever the earth has been torn 
open. From this point of view the collapse causes the 
fracture, the interior empty spaces cause the oscillatory 
motion, because the broken part of the crust is not sus- 
tained, and the volcanic eruption is but the result of 
this dislocation, because the molten matter enters the 
fissures only after their formation without having con- 
tributed to it. 

A similar revolution changes the distribution of the ocean : 
new continents rise from the bed of the sea, and others sink 
beneath the flood of waters. Then a fresh period of tran- 



GEOLOGICAL EPOCHS. 26 1 

quillity commences, until, the collapse continuing, similar 
phenomena are reproduced. 

The researches of Beaumont and of other geologists have 
shown us the traces of thirteen great revolutions and the 
order of their succession ; but we cannot know the intervals 
of time which have separated them ; we only know that 
they must be immense. Thus both geological and physical 
observations perfectly accord in proving the slowness of the 
earth's cooling. To these proofs we may add another, drawn 
from astronomical observations. 

Supposing that the sun has suffered no change, the con- 
traction of the earth should cause an acceleration of the 
motion of diurnal rotation, and, in consequence, diminish 
the length of the day. Now, according to Laplace, the 
length of the day has not decreased by ^ of a second in 
the last 2,000 years ; this effect is therefore inappreciable. 

The distribution of solar heat on the surface of the earth 
is, on the contrary, subject to great vicissitudes. It is pro- 
bable that the sun cools as slowly as the earth, and that the 
quantity of heat which it radiates to our earth is almost 
invariable ; but at each geological revolution this heat en- 
counters fresh continents and seas, and the climates are 
completely changed. Among the changes of this kind most 
nearly approaching the creation of man, we have a sinking 
of the whole of Switzerland, the traces of which have re- 
mained on the sides of the Alps and the Jura mountains. 

The traveller descending the valley of the Rhone may, if 
his knowledge be sufficient, trace the remains of ancient 
glaciers so far as the Lake of Geneva. Here the rocks 
which border the valley are scratched, and present deep 
furrows ; there they are polished and rounded off; in other 
places they are streaked and channeled out. They have 
everywhere the aspect of the rocks we may see bordering 
existing glaciers, where we may, in a measure, see the ice 
working under our eyes. What is much more, on the other 
side of the Lake of Geneva, on the chalky slopes of the 
Jura, rise blocks of the same granite which forms the 
summits of the Alps, as if these blocks had been detached 
from the top, and carried to this distance. We may see at 



262 THE PHENOMENA AND LAWS OF HEAT. 

the present time exactly similar effects in course of pro 
duction by the existing glaciers. Gliding slowly down its 
steep bed, the glacier breaks the rocks which oppose its 
progress, and drags the debris with it. They accumulate at 
certain points where the glacier deposits them in melting, 
and where consequently they form moraines. Everything 
seems to indicate that the valley of the Rhone was formerly 
an immense glacier, the moraines of which were formed on 
the plains of the Jura. Analogous traces are met with 
throughout Switzerland, England, the Lebanon range of 
Syria, and in North America. We must therefore admit 
that, at a certain epoch, these countries contained immense 
glaciers ; and hence the name of " the Glacial Epoch," given 
to a certain geological era. 

Will the supposition of a transient cooling, a diminution 
of the solar action, or the passage of the earth through 
excessively cold regions of the heavens, explain this epoch ? 
Tyndall has made a remark which has happily elucidated 
this question. Glaciers are the condensers of the ocean : 
to allow a large accumulation of ice on the mountains, the 
evaporation from the surface of the seas must be con- 
siderable, and therefore the sun must furnish more, rather 
than less, heat. To suppose that the glaciers augment in 
consequence of the suppression of the solar heat, is the 
same as trying to increase the distilling powers of a distil- 
latory apparatus, by diminishing the fire under the boiler. 
Solar heat could not therefore have been less active in the 
glacial epoch than at present. We have only to suppose, 
says Tyndall, a more powerful condensing apparatus than at 
present. For this it would be sufficient that the mountains 
were higher during the glacial epoch than they are now, 
because we know that the higher a mountain is, the colder 
is its summit. Switzerland and other countries where 
we find traces of ancient glaciers, have therefore suffered 
a slow sinking, due to the gradual sinking of the earth's 
crust ; and having thus become warmer, they have ceased to 
retain the atmospheric vapours as ice, their constant snows 
have been exchanged for rains, and their climate has been 
totally changed. 



PRIMITIVE STATE OF THE EARTH. 263 

The vicissitudes of climate have evidently accompanied 
those of the geographical situation of continents and seas ; 
and at each revolution certain species of animals and vege- 
tables have disappeared, the conditions necessary to their 
existence being no longer available, whilst new ones have 
appeared more adapted to the altered circumstances. We 
find in the strata deposited beneath the waters, at each fresh 
geological era, the debris of the species now lost, and we 
are enabled to compare them with the present species, and 
discover by this comparison the habits and diet of these 
mysterious beings. By deductions we are able to form an 
idea of the distribution of climates, and, although our inves- 
tigations lead only to simple conjectures, these may acquire, 
with the progress of science, an increasing degree of proba- 
bility. Thus, by reconstructing the earth and its inhabitants 
after each cataclysm, the geologist is able to show that the 
temperature was at first uniform over the whole surface of 
the globe, but that the climates were gradually modified, 
becoming at the same time more varied. 

Is not this result explained with the greatest simplicity 
by admitting that the earth was primitively liquid, and 
that the gradual solidification of its surface was produced 
by cooling? So long as the solid crust was but of slight 
thickness, the heat of the liquid centre was transmitted to 
the atmosphere by conduction, and the influence of seasons 
was insensible : later, when the thickness became greater, 
this influence became sensible. Each change in the quantity 
of water forming the sea, in the constitution of the atmo- 
sphere, in the elevation of a continent, brought about a 
change in the climate. Since the creation of man there 
has been no geological revolution comparable to those 
which preceded that epoch. It seems that the calorific 
state of the earth has become stationary, as if the solid 
crust had acquired sufficient thickness to no longer shrink 
up or break open, as in the past, and completely to inter- 
cept the central heat. 



264 THE PHENOMENA AND LAWS OF HEAT. 

4, Future of the terrestrial globe. 

The future destiny of the earth is a high question in na- 
tural philosophy which it is not our business to answer. We 
may nevertheless venture to give a hint of what may possibly 
happen. It is first necessary to distinguish between simple 
conjectures and observed facts. Among the latter, we have 
in the first place the great discoveries which form the science 
of geology. The time that has elapsed since the creation 
of man is incomparably less than the interval between two 
consecutive geological revolutions. Besides, the later geo- 
logical revolutions have effected fewer changes than the 
first in the conditions of the existence of organized beings. 
Having considered the past, we now come to the present. 
The same phenomena which have formerly accompanied 
the revolutions of the globe are still happening under our 
eyes. Add to these facts those which belong to the domain 
of physics. The sun and the earth are warmer than the 
celestial spaces, and they follow the laws of heat exactly as 
all other material bodies ; they must be continually cooling 
so long as they are at a higher temperature than the celestial 
spaces, although the most delicate instruments afford no 
indication of this phenomenon. Adding that astronomy 
confirms these observations, we shall have assembled before 
us the principal data of the problem. They are quite in- 
sufficient, however, to solve it, and if we go farther we 
immediately begin to hypothecate. If the earth is a liquid 
globe, covered by a thin solid skin, which sufficiently ex- 
plains the anterior geological phenomena, the same phe- 
nomena should be reproduced in successive centuries ; the 
speed of the diurnal rotation should increase, provided the 
sun and other celestial bodies continue to act in the same 
manner on our globe. The flattening at the poles should 
also increase, and the distribution of heat be thus changed ; 
there should be a greater difference between the mean tem- 
perature of the polar regions and that of the equatorial 
regions. Further, the orbit described by the earth around 
the sun should become lengthened, so that the winters would 
become colder and the summers hotter. Once in this vein, 



/ 



FUTURE OF THE TERRESTRIAL GLOBE. 265 

the mind knows no bounds ; obstacles seem to disappear 
as in a dream ; the possible seems to be almost without 
limit. But this is no more than a mirage or play of the 
imagination. A problem cannot be solved without com- 
plete data, but if we are unable to say to what extent the 
earth may be modified in future ages, it is not impossible 
to draw certain consequences from facts really observed. It 
is probable that a geological revolution will not come about 
within a period of time much exceeding that which has 
elapsed since the creation of man j and further, that the 
changes likely to occur will be less considerable than 
formerly. We conclude, therefore, that the present race of 
organized beings is less likely to be affected by geological 
revolutions than even those which characterized former 
epochs, and are now only known by their fossil remains. 

Fear is most often, if not always, the offspring of ignor- 
ance and superstition. Thank God, we live in an age when 
the light shed by science on the minds of the greater mass 
of the people is sufficient to save them from degrading 
terrors. Thunder and the sun's eclipses no longer affright 
civilized nations ; we contemplate them with a serenity born 
with a knowledge of the truth- and an admiration for the 
works of God ; and when nature's grander phenomena pre- 
sent themselves, we know that it is our duty to discover if 
any and what the peril is, that we may be prepared with 
all the resources furnished us by an all-wise Providence. 
Thus scientific' investigation fortifies within our soul the 
sentiment of adoration for the Divine Power, and raises us 
by degrees from the slavery of the physical to the freedom 
of the moral and spiritual world. Thus science and religion 
may truly be called sister spirits. 



THE END. 



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